diff --git a/readthedocs/projects/forms.py b/readthedocs/projects/forms.py
index 31e38ee81fa..fc2de9a7ad9 100644
--- a/readthedocs/projects/forms.py
+++ b/readthedocs/projects/forms.py
@@ -606,7 +606,15 @@ class DomainBaseForm(forms.ModelForm):
 
     class Meta:
         model = Domain
-        exclude = ['machine', 'cname', 'count']  # pylint: disable=modelform-uses-exclude
+        # pylint: disable=modelform-uses-exclude
+        exclude = [
+            'machine',
+            'cname',
+            'count',
+            'hsts_max_age',
+            'hsts_include_subdomains',
+            'hsts_preload',
+        ]
 
     def __init__(self, *args, **kwargs):
         self.project = kwargs.pop('project', None)
diff --git a/readthedocs/projects/migrations/0046_domain_hsts_fields.py b/readthedocs/projects/migrations/0046_domain_hsts_fields.py
new file mode 100644
index 00000000000..587b9dab5f9
--- /dev/null
+++ b/readthedocs/projects/migrations/0046_domain_hsts_fields.py
@@ -0,0 +1,28 @@
+# Generated by Django 2.2.12 on 2020-04-23 16:42
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('projects', '0045_project_max_concurrent_builds'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='domain',
+            name='hsts_include_subdomains',
+            field=models.BooleanField(default=False, help_text='If hsts_max_age > 0, set the includeSubDomains flag with the HSTS header'),
+        ),
+        migrations.AddField(
+            model_name='domain',
+            name='hsts_max_age',
+            field=models.PositiveIntegerField(default=0, help_text='Set a custom max-age (eg. 31536000) for the HSTS header'),
+        ),
+        migrations.AddField(
+            model_name='domain',
+            name='hsts_preload',
+            field=models.BooleanField(default=False, help_text='If hsts_max_age > 0, set the preload flag with the HSTS header'),
+        ),
+    ]
diff --git a/readthedocs/projects/models.py b/readthedocs/projects/models.py
index a5e1a9f6ad7..65af1bc7688 100644
--- a/readthedocs/projects/models.py
+++ b/readthedocs/projects/models.py
@@ -1448,6 +1448,22 @@ class Domain(models.Model):
         help_text=_('Number of times this domain has been hit'),
     )
 
+    # Strict-Transport-Security header options
+    # These are not exposed to users because it's easy to misconfigure things
+    # and hard to back out changes cleanly
+    hsts_max_age = models.PositiveIntegerField(
+        default=0,
+        help_text=_('Set a custom max-age (eg. 31536000) for the HSTS header')
+    )
+    hsts_include_subdomains = models.BooleanField(
+        default=False,
+        help_text=_('If hsts_max_age > 0, set the includeSubDomains flag with the HSTS header')
+    )
+    hsts_preload = models.BooleanField(
+        default=False,
+        help_text=_('If hsts_max_age > 0, set the preload flag with the HSTS header')
+    )
+
     objects = RelatedProjectQuerySet.as_manager()
 
     class Meta:
diff --git a/readthedocs/proxito/middleware.py b/readthedocs/proxito/middleware.py
index e30dc12eb99..26cc74825f3 100644
--- a/readthedocs/proxito/middleware.py
+++ b/readthedocs/proxito/middleware.py
@@ -82,6 +82,7 @@ def map_host_to_project_slug(request):  # pylint: disable=too-many-return-statem
     if domain:
         project_slug = domain.project.slug
         request.cname = True
+        request.domain = domain
         log.debug('Proxito CNAME: host=%s', host)
         return project_slug
 
@@ -114,3 +115,20 @@ def process_request(self, request):  # noqa
         request.host_project_slug = request.slug = ret
 
         return None
+
+    def process_response(self, request, response):  # noqa
+        """Set the Strict-Transport-Security (HSTS) header for a custom domain if max-age>0."""
+        if hasattr(request, 'domain'):
+            domain = request.domain
+            hsts_header_values = []
+            if domain.hsts_max_age:
+                hsts_header_values.append(f'max-age={domain.hsts_max_age}')
+                # These other options don't make sense without max_age > 0
+                if domain.hsts_include_subdomains:
+                    hsts_header_values.append('includeSubDomains')
+                if domain.hsts_preload:
+                    hsts_header_values.append('preload')
+
+                # See https://tools.ietf.org/html/rfc6797
+                response['Strict-Transport-Security'] = '; '.join(hsts_header_values)
+        return response
diff --git a/readthedocs/proxito/tests/test_full.py b/readthedocs/proxito/tests/test_full.py
index 2d91803e40d..f25b5e333fa 100644
--- a/readthedocs/proxito/tests/test_full.py
+++ b/readthedocs/proxito/tests/test_full.py
@@ -19,7 +19,7 @@
     SPHINX_HTMLDIR,
     SPHINX_SINGLEHTML,
 )
-from readthedocs.projects.models import Project
+from readthedocs.projects.models import Project, Domain
 from readthedocs.rtd_tests.storage import BuildMediaFileSystemStorageTest
 
 from .base import BaseDocServing
@@ -195,6 +195,39 @@ def test_valid_project_as_invalid_subproject(self):
         resp = self.client.get(url, HTTP_HOST=host)
         self.assertEqual(resp.status_code, 404)
 
+    def test_response_hsts(self):
+        hostname = 'docs.random.com'
+        domain = fixture.get(
+            Domain,
+            project=self.project,
+            domain=hostname,
+            hsts_max_age=0,
+            hsts_include_subdomains=False,
+            hsts_preload=False,
+        )
+
+        response = self.client.get("/", HTTP_HOST=hostname)
+        self.assertFalse('strict-transport-security' in response)
+
+        domain.hsts_max_age = 3600
+        domain.save()
+
+        response = self.client.get("/", HTTP_HOST=hostname)
+        self.assertTrue('strict-transport-security' in response)
+        self.assertEqual(
+            response['strict-transport-security'], 'max-age=3600',
+        )
+
+        domain.hsts_include_subdomains = True
+        domain.hsts_preload = True
+        domain.save()
+
+        response = self.client.get("/", HTTP_HOST=hostname)
+        self.assertTrue('strict-transport-security' in response)
+        self.assertEqual(
+            response['strict-transport-security'], 'max-age=3600; includeSubDomains; preload',
+        )
+
 
 class TestDocServingBackends(BaseDocServing):
     # Test that nginx and python backends both work