diff --git a/docs/api/v3.rst b/docs/api/v3.rst
index e4788083fc9..2c79cc12c6b 100644
--- a/docs/api/v3.rst
+++ b/docs/api/v3.rst
@@ -561,6 +561,34 @@ This allows for documentation projects to share a search index and a namespace o
 but still be maintained independently.
 See :doc:`/subprojects` for more information.
 
+
+Subproject details
+++++++++++++++++++
+
+
+.. http:get:: /api/v3/projects/(str:project_slug)/subprojects/(str:alias_slug)/
+
+    Retrieve details of a subproject relationship.
+
+    **Example request**:
+
+    .. sourcecode:: bash
+
+        $ curl -H "Authorization: Token <token>" https://readthedocs.org/api/v3/projects/pip/subprojects/subproject-alias/
+
+    **Example response**:
+
+    .. sourcecode:: json
+
+        {
+            "alias": "subproject-alias",
+            "child": ["PROJECT"],
+            "_links": {
+                "parent": "/api/v3/projects/pip/"
+            }
+        }
+
+
 Subprojects listing
 +++++++++++++++++++
 
@@ -583,15 +611,64 @@ Subprojects listing
             "count": 25,
             "next": "/api/v3/projects/pip/subprojects/?limit=10&offset=10",
             "previous": null,
-            "results": ["PROJECT"]
+            "results": ["SUBPROJECT RELATIONSHIP"]
         }
 
-    :>json integer count: total number of projects.
-    :>json string next: URI for next set of projects.
-    :>json string previous: URI for previous set of projects.
-    :>json array results: array of ``project`` objects.
 
-    :requestheader Authorization: token to authenticate.
+Subproject create
++++++++++++++++++
+
+
+.. http:post:: /api/v3/projects/(str:project_slug)/subprojects/
+
+    Create a subproject relationship between two projects.
+
+    **Example request**:
+
+    .. sourcecode:: bash
+
+        $ curl \
+          -X POST \
+          -H "Authorization: Token <token>" https://readthedocs.org/api/v3/projects/pip/subprojects/ \
+          -H "Content-Type: application/json" \
+          -d @body.json
+
+    The content of ``body.json`` is like,
+
+    .. sourcecode:: json
+
+        {
+            "child": "subproject-child-slug",
+            "alias": "subproject-alias"
+        }
+
+    **Example response**:
+
+    `See Subproject details <#subproject-details>`_
+
+    :>json string child: slug of the child project in the relationship.
+    :>json string alias: optional slug alias to be used in the URL (e.g ``/projects/<alias>/en/latest/``).
+                         If not provided, child project's slug is used as alias.
+
+    :statuscode 201: Subproject created sucessfully
+
+
+Subproject delete
++++++++++++++++++
+
+.. http:delete:: /api/v3/projects/(str:project_slug)/subprojects/(str:alias_slug)/
+
+    Delete a subproject relationship.
+
+    **Example request**:
+
+    .. sourcecode:: bash
+
+        $ curl \
+          -X DELETE \
+          -H "Authorization: Token <token>" https://readthedocs.org/api/v3/projects/pip/subprojects/subproject-alias/
+
+    :statuscode 204: Subproject deleted successfully
 
 
 Translations
diff --git a/readthedocs/api/v3/mixins.py b/readthedocs/api/v3/mixins.py
index bd608d1a793..21c152778fa 100644
--- a/readthedocs/api/v3/mixins.py
+++ b/readthedocs/api/v3/mixins.py
@@ -13,6 +13,7 @@ class NestedParentObjectMixin:
     PROJECT_LOOKUP_NAMES = [
         'project__slug',
         'projects__slug',
+        'parent__slug',
         'superprojects__parent__slug',
         'main_language_project__slug',
     ]
diff --git a/readthedocs/api/v3/serializers.py b/readthedocs/api/v3/serializers.py
index 44a90721e33..992de2cbace 100644
--- a/readthedocs/api/v3/serializers.py
+++ b/readthedocs/api/v3/serializers.py
@@ -18,7 +18,7 @@
     PRIVACY_CHOICES,
     PROTECTED,
 )
-from readthedocs.projects.models import Project, EnvironmentVariable
+from readthedocs.projects.models import Project, EnvironmentVariable, ProjectRelationship
 from readthedocs.redirects.models import Redirect, TYPE_CHOICES as REDIRECT_TYPE_CHOICES
 
 
@@ -385,7 +385,7 @@ def get_subprojects(self, obj):
         path = reverse(
             'projects-subprojects-list',
             kwargs={
-                'parent_lookup_superprojects__parent__slug': obj.slug,
+                'parent_lookup_parent__slug': obj.slug,
             },
         )
         return self._absolute_url(path)
@@ -539,6 +539,128 @@ def get_subproject_of(self, obj):
             return None
 
 
+class SubprojectCreateSerializer(FlexFieldsModelSerializer):
+
+    """Serializer used to define a Project as subproject of another Project."""
+
+    child = serializers.SlugRelatedField(
+        slug_field='slug',
+        queryset=Project.objects.all(),
+    )
+
+    class Meta:
+        model = ProjectRelationship
+        fields = [
+            'child',
+            'alias',
+        ]
+
+    def __init__(self, *args, **kwargs):
+        # Initialize the instance with the parent Project to be used in the
+        # serializer validation.
+        self.parent_project = kwargs.pop('parent')
+        super().__init__(*args, **kwargs)
+
+    def validate_child(self, value):
+        # Check the user is maintainer of the child project
+        user = self.context['request'].user
+        if user not in value.users.all():
+            raise serializers.ValidationError(
+                'You do not have permissions on the child project',
+            )
+        return value
+
+    def validate_alias(self, value):
+        # Check there is not a subproject with this alias already
+        subproject = self.parent_project.subprojects.filter(alias=value)
+        if subproject.exists():
+            raise serializers.ValidationError(
+                'A subproject with this alias already exists',
+            )
+        return value
+
+    # pylint: disable=arguments-differ
+    def validate(self, data):
+        # Check the parent and child are not the same project
+        if data['child'].slug == self.parent_project.slug:
+            raise serializers.ValidationError(
+                'Project can not be subproject of itself',
+            )
+
+        # Check the parent project is not a subproject already
+        if self.parent_project.superprojects.exists():
+            raise serializers.ValidationError(
+                'Subproject nesting is not supported',
+            )
+        return data
+
+
+class SubprojectLinksSerializer(BaseLinksSerializer):
+    _self = serializers.SerializerMethodField()
+    parent = serializers.SerializerMethodField()
+
+    def get__self(self, obj):
+        path = reverse(
+            'projects-subprojects-detail',
+            kwargs={
+                'parent_lookup_parent__slug': obj.parent.slug,
+                'alias_slug': obj.alias,
+            },
+        )
+        return self._absolute_url(path)
+
+    def get_parent(self, obj):
+        path = reverse(
+            'projects-detail',
+            kwargs={
+                'project_slug': obj.parent.slug,
+            },
+        )
+        return self._absolute_url(path)
+
+
+class ChildProjectSerializer(ProjectSerializer):
+
+    """
+    Serializer to render a Project when listed under ProjectRelationship.
+
+    It's exactly the same as ``ProjectSerializer`` but without some fields.
+    """
+
+    class Meta(ProjectSerializer.Meta):
+        fields = [
+            field for field in ProjectSerializer.Meta.fields
+            if field not in ['subproject_of']
+        ]
+
+
+class SubprojectSerializer(FlexFieldsModelSerializer):
+
+    """Serializer to render a subproject (``ProjectRelationship``)."""
+
+    child = ChildProjectSerializer()
+    _links = SubprojectLinksSerializer(source='*')
+
+    class Meta:
+        model = ProjectRelationship
+        fields = [
+            'child',
+            'alias',
+            '_links',
+        ]
+
+
+class SubprojectDestroySerializer(FlexFieldsModelSerializer):
+
+    """Serializer used to remove a subproject relationship to a Project."""
+
+    class Meta:
+        model = ProjectRelationship
+        fields = (
+            'alias',
+        )
+
+
 class RedirectLinksSerializer(BaseLinksSerializer):
     _self = serializers.SerializerMethodField()
     project = serializers.SerializerMethodField()
diff --git a/readthedocs/api/v3/tests/mixins.py b/readthedocs/api/v3/tests/mixins.py
index 263aaf7bc59..d3ff2682f14 100644
--- a/readthedocs/api/v3/tests/mixins.py
+++ b/readthedocs/api/v3/tests/mixins.py
@@ -60,22 +60,6 @@ def setUp(self):
             project=self.project,
         )
 
-        self.subproject = fixture.get(
-            Project,
-            pub_date=self.created,
-            modified_date=self.modified,
-            description='SubProject description',
-            repo='https://github.com/rtfd/subproject',
-            project_url='http://subproject.com',
-            name='subproject',
-            slug='subproject',
-            related_projects=[],
-            main_language_project=None,
-            users=[],
-            versions=[],
-        )
-        self.project.add_subproject(self.subproject)
-
         self.version = fixture.get(
             Version,
             slug='v1.0',
@@ -119,6 +103,41 @@ def tearDown(self):
         # Cleanup cache to avoid throttling on tests
         cache.clear()
 
+    def _create_new_project(self):
+        """Helper to create a project with all the fields set."""
+        return fixture.get(
+            Project,
+            pub_date=self.created,
+            modified_date=self.modified,
+            description='Project description',
+            repo='https://github.com/rtfd/project',
+            project_url='http://project.com',
+            name='new-project',
+            slug='new-project',
+            related_projects=[],
+            main_language_project=None,
+            users=[self.me],
+            versions=[],
+        )
+
+    def _create_subproject(self):
+        """Helper to create a sub-project with all the fields set."""
+        self.subproject = fixture.get(
+            Project,
+            pub_date=self.created,
+            modified_date=self.modified,
+            description='SubProject description',
+            repo='https://github.com/rtfd/subproject',
+            project_url='http://subproject.com',
+            name='subproject',
+            slug='subproject',
+            related_projects=[],
+            main_language_project=None,
+            users=[self.me],
+            versions=[],
+        )
+        self.project_relationship = self.project.add_subproject(self.subproject)
+
     def _get_response_dict(self, view_name):
         filename = Path(__file__).absolute().parent / 'responses' / f'{view_name}.json'
         return json.load(open(filename))
diff --git a/readthedocs/api/v3/tests/responses/projects-detail.json b/readthedocs/api/v3/tests/responses/projects-detail.json
index dafbee8d1b5..a5f61e9c112 100644
--- a/readthedocs/api/v3/tests/responses/projects-detail.json
+++ b/readthedocs/api/v3/tests/responses/projects-detail.json
@@ -4,7 +4,7 @@
             "active": true,
             "built": true,
             "downloads": {},
-            "id": 3,
+            "id": 2,
             "identifier": "a1b2c3",
             "last_build": {
                 "commit": "a1b2c3",
diff --git a/readthedocs/api/v3/tests/responses/projects-list_POST.json b/readthedocs/api/v3/tests/responses/projects-list_POST.json
index 20ea4ce9edc..1bcb04caba4 100644
--- a/readthedocs/api/v3/tests/responses/projects-list_POST.json
+++ b/readthedocs/api/v3/tests/responses/projects-list_POST.json
@@ -12,7 +12,7 @@
     "created": "2019-04-29T10:00:00Z",
     "default_branch": "master",
     "default_version": "latest",
-    "id": 4,
+    "id": 3,
     "homepage": "http://template.readthedocs.io/",
     "language": {
         "code": "en",
diff --git a/readthedocs/api/v3/tests/responses/projects-subprojects-detail.json b/readthedocs/api/v3/tests/responses/projects-subprojects-detail.json
new file mode 100644
index 00000000000..2966e4c0eb7
--- /dev/null
+++ b/readthedocs/api/v3/tests/responses/projects-subprojects-detail.json
@@ -0,0 +1,56 @@
+{
+    "_links": {
+        "_self": "https://readthedocs.org/api/v3/projects/project/subprojects/subproject/",
+        "parent": "https://readthedocs.org/api/v3/projects/project/"
+    },
+    "alias": "subproject",
+    "child": {
+        "_links": {
+            "_self": "https://readthedocs.org/api/v3/projects/subproject/",
+            "builds": "https://readthedocs.org/api/v3/projects/subproject/builds/",
+            "environmentvariables": "https://readthedocs.org/api/v3/projects/subproject/environmentvariables/",
+            "redirects": "https://readthedocs.org/api/v3/projects/subproject/redirects/",
+            "subprojects": "https://readthedocs.org/api/v3/projects/subproject/subprojects/",
+            "superproject": "https://readthedocs.org/api/v3/projects/subproject/superproject/",
+            "translations": "https://readthedocs.org/api/v3/projects/subproject/translations/",
+            "versions": "https://readthedocs.org/api/v3/projects/subproject/versions/"
+        },
+        "created": "2019-04-29T10:00:00Z",
+        "default_branch": "master",
+        "default_version": "latest",
+        "homepage": "http://subproject.com",
+        "id": 3,
+        "language": {
+            "code": "en",
+            "name": "English"
+        },
+        "modified": "2019-04-29T12:00:00Z",
+        "name": "subproject",
+        "privacy_level": {
+            "code": "public",
+            "name": "Public"
+        },
+        "programming_language": {
+            "code": "words",
+            "name": "Only Words"
+        },
+        "repository": {
+            "type": "git",
+            "url": "https://github.com/rtfd/subproject"
+        },
+        "slug": "subproject",
+        "tags": [],
+        "translation_of": null,
+        "urls": {
+            "builds": "https://readthedocs.org/projects/subproject/builds/",
+            "documentation": "http://readthedocs.org/docs/project/projects/subproject/en/latest/",
+            "home": "https://readthedocs.org/projects/subproject/",
+            "versions": "https://readthedocs.org/projects/subproject/versions/"
+        },
+        "users": [
+            {
+                "username": "testuser"
+            }
+        ]
+    }
+}
diff --git a/readthedocs/api/v3/tests/responses/projects-subprojects-list.json b/readthedocs/api/v3/tests/responses/projects-subprojects-list.json
index e5df1382523..79c2f06c54d 100644
--- a/readthedocs/api/v3/tests/responses/projects-subprojects-list.json
+++ b/readthedocs/api/v3/tests/responses/projects-subprojects-list.json
@@ -4,102 +4,59 @@
     "previous": null,
     "results": [
         {
-            "id": 2,
-            "name": "subproject",
-            "slug": "subproject",
-            "created": "2019-04-29T10:00:00Z",
-            "modified": "2019-04-29T12:00:00Z",
-            "language": {
-                "code": "en",
-                "name": "English"
-            },
-            "programming_language": {
-                "code": "words",
-                "name": "Only Words"
-            },
-            "homepage": "http://subproject.com",
-            "repository": {
-                "url": "https://github.com/rtfd/subproject",
-                "type": "git"
-            },
-            "default_version": "latest",
-            "default_branch": "master",
-            "privacy_level": {
-                "code": "public",
-                "name": "Public"
+            "_links": {
+                "_self": "https://readthedocs.org/api/v3/projects/project/subprojects/subproject/",
+                "parent": "https://readthedocs.org/api/v3/projects/project/"
             },
-            "subproject_of": {
-                "id": 1,
-                "name": "project",
-                "slug": "project",
+            "alias": "subproject",
+            "child": {
+                "_links": {
+                    "_self": "https://readthedocs.org/api/v3/projects/subproject/",
+                    "builds": "https://readthedocs.org/api/v3/projects/subproject/builds/",
+                    "environmentvariables": "https://readthedocs.org/api/v3/projects/subproject/environmentvariables/",
+                    "redirects": "https://readthedocs.org/api/v3/projects/subproject/redirects/",
+                    "subprojects": "https://readthedocs.org/api/v3/projects/subproject/subprojects/",
+                    "superproject": "https://readthedocs.org/api/v3/projects/subproject/superproject/",
+                    "translations": "https://readthedocs.org/api/v3/projects/subproject/translations/",
+                    "versions": "https://readthedocs.org/api/v3/projects/subproject/versions/"
+                },
                 "created": "2019-04-29T10:00:00Z",
-                "modified": "2019-04-29T12:00:00Z",
+                "default_branch": "master",
+                "default_version": "latest",
+                "homepage": "http://subproject.com",
+                "id": 3,
                 "language": {
                     "code": "en",
                     "name": "English"
                 },
+                "modified": "2019-04-29T12:00:00Z",
+                "name": "subproject",
+                "privacy_level": {
+                    "code": "public",
+                    "name": "Public"
+                },
                 "programming_language": {
                     "code": "words",
                     "name": "Only Words"
                 },
-                "homepage": "http://project.com",
                 "repository": {
-                    "url": "https://github.com/rtfd/project",
-                    "type": "git"
-                },
-                "default_version": "latest",
-                "default_branch": "master",
-                "privacy_level": {
-                    "code": "public",
-                    "name": "Public"
+                    "type": "git",
+                    "url": "https://github.com/rtfd/subproject"
                 },
-                "subproject_of": null,
+                "slug": "subproject",
+                "tags": [],
                 "translation_of": null,
                 "urls": {
-                    "builds": "https://readthedocs.org/projects/project/builds/",
-                    "documentation": "http://readthedocs.org/docs/project/en/latest/",
-                    "home": "https://readthedocs.org/projects/project/",
-                    "versions": "https://readthedocs.org/projects/project/versions/"
-                },
-                "tags": [
-                    "tag",
-                    "project",
-                    "test"
-                ],
-                "_links": {
-                    "_self": "https://readthedocs.org/api/v3/projects/project/",
-                    "versions": "https://readthedocs.org/api/v3/projects/project/versions/",
-                    "builds": "https://readthedocs.org/api/v3/projects/project/builds/",
-                    "environmentvariables": "https://readthedocs.org/api/v3/projects/project/environmentvariables/",
-                    "redirects": "https://readthedocs.org/api/v3/projects/project/redirects/",
-                    "subprojects": "https://readthedocs.org/api/v3/projects/project/subprojects/",
-                    "superproject": "https://readthedocs.org/api/v3/projects/project/superproject/",
-                    "translations": "https://readthedocs.org/api/v3/projects/project/translations/"
+                    "builds": "https://readthedocs.org/projects/subproject/builds/",
+                    "documentation": "http://readthedocs.org/docs/project/projects/subproject/en/latest/",
+                    "home": "https://readthedocs.org/projects/subproject/",
+                    "versions": "https://readthedocs.org/projects/subproject/versions/"
                 },
                 "users": [
                     {
                         "username": "testuser"
                     }
                 ]
-            },
-            "translation_of": null,
-            "urls": {
-                "builds": "https://readthedocs.org/projects/subproject/builds/",
-                "documentation": "http://readthedocs.org/docs/project/projects/subproject/en/latest/",
-                "home": "https://readthedocs.org/projects/subproject/",
-                "versions": "https://readthedocs.org/projects/subproject/versions/"
-            },
-            "tags": [],
-            "users": [],
-            "_links": {
-                "_self": "https://readthedocs.org/api/v3/projects/subproject/",
-                "versions": "https://readthedocs.org/api/v3/projects/subproject/versions/",
-                "builds": "https://readthedocs.org/api/v3/projects/subproject/builds/",
-                "environmentvariables": "https://readthedocs.org/api/v3/projects/subproject/environmentvariables/",
-                "redirects": "https://readthedocs.org/api/v3/projects/subproject/redirects/",
-                "subprojects": "https://readthedocs.org/api/v3/projects/subproject/subprojects/",
-                "superproject": "https://readthedocs.org/api/v3/projects/subproject/superproject/",
-                "translations": "https://readthedocs.org/api/v3/projects/subproject/translations/"
             }
         }
     ]
diff --git a/readthedocs/api/v3/tests/responses/projects-subprojects-list_POST.json b/readthedocs/api/v3/tests/responses/projects-subprojects-list_POST.json
new file mode 100644
index 00000000000..1a02625f3a2
--- /dev/null
+++ b/readthedocs/api/v3/tests/responses/projects-subprojects-list_POST.json
@@ -0,0 +1,56 @@
+{
+    "_links": {
+        "_self": "https://readthedocs.org/api/v3/projects/project/subprojects/subproject-alias/",
+        "parent": "https://readthedocs.org/api/v3/projects/project/"
+    },
+    "alias": "subproject-alias",
+    "child": {
+        "_links": {
+            "_self": "https://readthedocs.org/api/v3/projects/new-project/",
+            "builds": "https://readthedocs.org/api/v3/projects/new-project/builds/",
+            "environmentvariables": "https://readthedocs.org/api/v3/projects/new-project/environmentvariables/",
+            "redirects": "https://readthedocs.org/api/v3/projects/new-project/redirects/",
+            "subprojects": "https://readthedocs.org/api/v3/projects/new-project/subprojects/",
+            "superproject": "https://readthedocs.org/api/v3/projects/new-project/superproject/",
+            "translations": "https://readthedocs.org/api/v3/projects/new-project/translations/",
+            "versions": "https://readthedocs.org/api/v3/projects/new-project/versions/"
+        },
+        "created": "2019-04-29T10:00:00Z",
+        "default_branch": "master",
+        "default_version": "latest",
+        "homepage": "http://project.com",
+        "id": 4,
+        "language": {
+            "code": "en",
+            "name": "English"
+        },
+        "modified": "2019-04-29T12:00:00Z",
+        "name": "new-project",
+        "privacy_level": {
+            "code": "public",
+            "name": "Public"
+        },
+        "programming_language": {
+            "code": "words",
+            "name": "Only Words"
+        },
+        "repository": {
+            "type": "git",
+            "url": "https://github.com/rtfd/project"
+        },
+        "slug": "new-project",
+        "tags": [],
+        "translation_of": null,
+        "urls": {
+            "builds": "https://readthedocs.org/projects/new-project/builds/",
+            "documentation": "http://readthedocs.org/docs/project/projects/subproject-alias/en/latest/",
+            "home": "https://readthedocs.org/projects/new-project/",
+            "versions": "https://readthedocs.org/projects/new-project/versions/"
+        },
+        "users": [
+            {
+                "username": "testuser"
+            }
+        ]
+    }
+}
diff --git a/readthedocs/api/v3/tests/responses/projects-versions-builds-list_POST.json b/readthedocs/api/v3/tests/responses/projects-versions-builds-list_POST.json
index 6f099147022..088b49ef2c8 100644
--- a/readthedocs/api/v3/tests/responses/projects-versions-builds-list_POST.json
+++ b/readthedocs/api/v3/tests/responses/projects-versions-builds-list_POST.json
@@ -78,7 +78,7 @@
         "active": true,
         "built": true,
         "downloads": {},
-        "id": 3,
+        "id": 2,
         "identifier": "a1b2c3",
         "_links": {
             "_self": "https://readthedocs.org/api/v3/projects/project/versions/v1.0/",
diff --git a/readthedocs/api/v3/tests/responses/projects-versions-detail.json b/readthedocs/api/v3/tests/responses/projects-versions-detail.json
index 861d951f36e..bb488a1f1c4 100644
--- a/readthedocs/api/v3/tests/responses/projects-versions-detail.json
+++ b/readthedocs/api/v3/tests/responses/projects-versions-detail.json
@@ -2,7 +2,7 @@
     "active": true,
     "built": true,
     "downloads": {},
-    "id": 3,
+    "id": 2,
     "identifier": "a1b2c3",
     "_links": {
         "_self": "https://readthedocs.org/api/v3/projects/project/versions/v1.0/",
diff --git a/readthedocs/api/v3/tests/test_projects.py b/readthedocs/api/v3/tests/test_projects.py
index a74aad0ba83..447fbadd6be 100644
--- a/readthedocs/api/v3/tests/test_projects.py
+++ b/readthedocs/api/v3/tests/test_projects.py
@@ -41,6 +41,7 @@ def test_own_projects_detail(self):
         )
 
     def test_projects_superproject(self):
+        self._create_subproject()
         self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
         response = self.client.get(
             reverse(
@@ -57,22 +58,6 @@ def test_projects_superproject(self):
             self._get_response_dict('projects-superproject'),
         )
 
-    def test_projects_subprojects_list(self):
-        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
-        response = self.client.get(
-            reverse(
-                'projects-subprojects-list',
-                kwargs={
-                    'parent_lookup_superprojects__parent__slug': self.project.slug,
-                },
-            ),
-        )
-        self.assertEqual(response.status_code, 200)
-        self.assertDictEqual(
-            response.json(),
-            self._get_response_dict('projects-subprojects-list'),
-        )
-
     def test_others_projects_builds_list(self):
         self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
         response = self.client.get(
diff --git a/readthedocs/api/v3/tests/test_subprojects.py b/readthedocs/api/v3/tests/test_subprojects.py
new file mode 100644
index 00000000000..460adf32ef9
--- /dev/null
+++ b/readthedocs/api/v3/tests/test_subprojects.py
@@ -0,0 +1,211 @@
+from .mixins import APIEndpointMixin
+from django.urls import reverse
+import django_dynamic_fixture as fixture
+
+from readthedocs.projects.models import Project
+
+
+class SubprojectsEndpointTests(APIEndpointMixin):
+
+    def setUp(self):
+        super().setUp()
+        self._create_subproject()
+
+    def test_projects_subprojects_list(self):
+        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
+        response = self.client.get(
+            reverse(
+                'projects-subprojects-list',
+                kwargs={
+                    'parent_lookup_parent__slug': self.project.slug,
+                },
+            ),
+        )
+        self.assertEqual(response.status_code, 200)
+        self.assertDictEqual(
+            response.json(),
+            self._get_response_dict('projects-subprojects-list'),
+        )
+
+    def test_projects_subprojects_detail(self):
+        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
+        response = self.client.get(
+            reverse(
+                'projects-subprojects-detail',
+                kwargs={
+                    'parent_lookup_parent__slug': self.project.slug,
+                    'alias_slug': self.project_relationship.alias,
+                }),
+        )
+        self.assertEqual(response.status_code, 200)
+        self.assertDictEqual(
+            response.json(),
+            self._get_response_dict('projects-subprojects-detail'),
+        )
+
+    def test_projects_subprojects_list_post(self):
+        newproject = self._create_new_project()
+        self.assertEqual(self.project.subprojects.count(), 1)
+        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
+        data = {
+            'child': newproject.slug,
+            'alias': 'subproject-alias',
+        }
+        response = self.client.post(
+            reverse(
+                'projects-subprojects-list',
+                kwargs={
+                    'parent_lookup_parent__slug': self.project.slug,
+                },
+            ),
+            data,
+        )
+
+        self.assertEqual(response.status_code, 201)
+        self.assertEqual(self.project.subprojects.count(), 2)
+
+        self.assertDictEqual(
+            response.json(),
+            self._get_response_dict('projects-subprojects-list_POST'),
+        )
+
+    def test_projects_subprojects_list_post_with_others_as_child(self):
+        self.assertEqual(self.project.subprojects.count(), 1)
+        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
+        data = {
+            'child': self.others_project.slug,
+            'alias': 'subproject-alias',
+        }
+        response = self.client.post(
+            reverse(
+                'projects-subprojects-list',
+                kwargs={
+                    'parent_lookup_parent__slug': self.project.slug,
+                },
+            ),
+            data,
+        )
+        self.assertEqual(response.status_code, 400)
+        self.assertEqual(self.project.subprojects.count(), 1)
+
+    def test_projects_subprojects_list_post_with_subproject_of_itself(self):
+        self.assertEqual(self.project.subprojects.count(), 1)
+        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
+        data = {
+            'child': self.project.slug,
+            'alias': 'subproject-alias',
+        }
+        response = self.client.post(
+            reverse(
+                'projects-subprojects-list',
+                kwargs={
+                    'parent_lookup_parent__slug': self.project.slug,
+                },
+            ),
+            data,
+        )
+        self.assertEqual(response.status_code, 400)
+        self.assertIn(
+            'Project can not be subproject of itself',
+            response.json()['non_field_errors'],
+        )
+        self.assertEqual(self.project.subprojects.count(), 1)
+
+    def test_projects_subprojects_list_post_nested_subproject(self):
+        newproject = self._create_new_project()
+        self.assertEqual(self.project.subprojects.count(), 1)
+        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
+        data = {
+            'child': newproject.slug,
+            'alias': 'subproject-alias',
+        }
+        response = self.client.post(
+            reverse(
+                'projects-subprojects-list',
+                kwargs={
+                    'parent_lookup_parent__slug': self.subproject.slug,
+                },
+            ),
+            data,
+        )
+        self.assertEqual(response.status_code, 400)
+        self.assertIn(
+            'Subproject nesting is not supported',
+            response.json()['non_field_errors'],
+        )
+        self.assertEqual(self.project.subprojects.count(), 1)
+
+    def test_projects_subprojects_list_post_unique_alias(self):
+        newproject = self._create_new_project()
+        self.assertEqual(self.project.subprojects.count(), 1)
+        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
+        data = {
+            'child': newproject.slug,
+            'alias': 'subproject',  # this alias is already set for another subproject
+        }
+        response = self.client.post(
+            reverse(
+                'projects-subprojects-list',
+                kwargs={
+                    'parent_lookup_parent__slug': self.project.slug,
+                },
+            ),
+            data,
+        )
+        self.assertEqual(response.status_code, 400)
+        self.assertIn(
+            'A subproject with this alias already exists',
+            response.json()['alias'],
+        )
+        self.assertEqual(self.project.subprojects.count(), 1)
+
+    def test_projects_subprojects_list_post_with_others_as_parent(self):
+        self.assertEqual(self.others_project.subprojects.count(), 0)
+        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
+        data = {
+            'child': self.project.slug,
+            'alias': 'subproject-alias',
+        }
+        response = self.client.post(
+            reverse(
+                'projects-subprojects-list',
+                kwargs={
+                    'parent_lookup_parent__slug': self.others_project.slug,
+                },
+            ),
+            data,
+        )
+        self.assertEqual(response.status_code, 403)
+        self.assertEqual(self.others_project.subprojects.count(), 0)
+
+    def test_projects_subprojects_detail_delete(self):
+        self.assertEqual(self.project.subprojects.count(), 1)
+        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
+        response = self.client.delete(
+            reverse(
+                'projects-subprojects-detail',
+                kwargs={
+                    'parent_lookup_parent__slug': self.project.slug,
+                    'alias_slug': self.project_relationship.alias,
+                },
+            ),
+        )
+        self.assertEqual(response.status_code, 204)
+        self.assertEqual(self.project.subprojects.count(), 0)
+
+    def test_projects_subprojects_detail_delete_others_project(self):
+        newproject =  self._create_new_project()
+        project_relationship = self.others_project.add_subproject(newproject)
+        self.assertEqual(self.others_project.subprojects.count(), 1)
+        self.client.credentials(HTTP_AUTHORIZATION=f'Token {self.token.key}')
+        response = self.client.delete(
+            reverse(
+                'projects-subprojects-detail',
+                kwargs={
+                    'parent_lookup_parent__slug': self.others_project.slug,
+                    'alias_slug': project_relationship.alias,
+                },
+            ),
+        )
+        self.assertEqual(response.status_code, 403)
+        self.assertEqual(self.project.subprojects.count(), 1)
diff --git a/readthedocs/api/v3/urls.py b/readthedocs/api/v3/urls.py
index fb0d41b5981..c52da4fb062 100644
--- a/readthedocs/api/v3/urls.py
+++ b/readthedocs/api/v3/urls.py
@@ -27,7 +27,7 @@
     r'subprojects',
     SubprojectRelationshipViewSet,
     basename='projects-subprojects',
-    parents_query_lookups=['superprojects__parent__slug'],
+    parents_query_lookups=['parent__slug'],
 )
 
 # allows /api/v3/projects/pip/translations/
diff --git a/readthedocs/api/v3/views.py b/readthedocs/api/v3/views.py
index 7d863a0b20e..332bf517aca 100644
--- a/readthedocs/api/v3/views.py
+++ b/readthedocs/api/v3/views.py
@@ -21,7 +21,7 @@
 
 from readthedocs.builds.models import Build, Version
 from readthedocs.core.utils import trigger_build
-from readthedocs.projects.models import Project, EnvironmentVariable
+from readthedocs.projects.models import Project, EnvironmentVariable, ProjectRelationship
 from readthedocs.projects.views.mixins import ProjectImportMixin
 from readthedocs.redirects.models import Redirect
 
@@ -39,6 +39,9 @@
     ProjectUpdateSerializer,
     RedirectCreateSerializer,
     RedirectDetailSerializer,
+    SubprojectCreateSerializer,
+    SubprojectSerializer,
+    SubprojectDestroySerializer,
     VersionSerializer,
     VersionUpdateSerializer,
 )
@@ -231,11 +234,12 @@ def superproject(self, request, project_slug):
 
 class SubprojectRelationshipViewSet(APIv3Settings, NestedViewSetMixin,
                                     ProjectQuerySetMixin, FlexFieldsMixin,
-                                    ListModelMixin, GenericViewSet):
+                                    CreateModelMixin, DestroyModelMixin,
+                                    ReadOnlyModelViewSet):
 
     # Markdown docstring exposed at BrowsableAPIRenderer.
 
-    """List subprojects of a ``Project``."""
+    """List subprojects (``ProjectRelationship``) of a ``Project``."""
 
     # Private/Internal docstring
 
@@ -244,11 +248,38 @@ class SubprojectRelationshipViewSet(APIv3Settings, NestedViewSetMixin,
     ``parents_query_lookups`` defined when registering the urls.
     """  # noqa
 
-    model = Project
-    lookup_field = 'slug'
-    lookup_url_kwarg = 'project_slug'
-    serializer_class = ProjectSerializer
-    queryset = Project.objects.all()
+    model = ProjectRelationship
+    lookup_field = 'alias'
+    lookup_url_kwarg = 'alias_slug'
+    queryset = ProjectRelationship.objects.all()
+
+    def get_serializer_class(self):
+        """
+        Return correct serializer depending on the action.
+
+        For GET it returns a serializer with many fields and on POST,
+        it return a serializer to validate just a few fields.
+        """
+        if self.action == 'create':
+            return SubprojectCreateSerializer
+
+        if self.action == 'destroy':
+            return SubprojectDestroySerializer
+
+        return SubprojectSerializer
+
+    def create(self, request, *args, **kwargs):
+        """Define a Project as subproject of another Project."""
+        parent = self._get_parent_project()
+        serializer = self.get_serializer(parent=parent, data=request.data)
+        serializer.is_valid(raise_exception=True)
+        serializer.save(parent=parent)
+        headers = self.get_success_headers(serializer.data)
+
+        # Use serializer that fully render a the subproject
+        serializer = SubprojectSerializer(instance=serializer.instance)
+
+        return Response(serializer.data, status=status.HTTP_201_CREATED, headers=headers)
 
 
 class TranslationRelationshipViewSet(APIv3Settings, NestedViewSetMixin,
diff --git a/readthedocs/projects/querysets.py b/readthedocs/projects/querysets.py
index efd01c8026c..22d87d1eeee 100644
--- a/readthedocs/projects/querysets.py
+++ b/readthedocs/projects/querysets.py
@@ -137,7 +137,8 @@ def _add_user_repos(self, queryset, user=None):
             return self.all()
         if user.is_authenticated:
             projects_pk = user.projects.all().values_list('pk', flat=True)
-            user_queryset = self.filter(project__in=projects_pk)
+            kwargs = {f'{self.project_field}__in': projects_pk}
+            user_queryset = self.filter(**kwargs)
             queryset = user_queryset | queryset
         return queryset