diff --git a/docs/guides/feature-flags.rst b/docs/guides/feature-flags.rst
index 9beb7043aa9..3976421a6cd 100644
--- a/docs/guides/feature-flags.rst
+++ b/docs/guides/feature-flags.rst
@@ -37,6 +37,8 @@ e.g. python-reno release notes manager is known to do that
 
 ``EXTERNAL_VERSION_BUILD``: :featureflags:`EXTERNAL_VERSION_BUILD`
 
+``LIST_PACKAGES_INSTALLED_ENV``: :featureflags:`LIST_PACKAGES_INSTALLED_ENV`
+
 ``SHARE_SPHINX_DOCTREE``: :featureflags:`SHARE_SPHINX_DOCTREE`
 
 By default, when Read the Docs runs Sphinx it passes a different output directory for the generated/parsed doctrees
diff --git a/readthedocs/doc_builder/python_environments.py b/readthedocs/doc_builder/python_environments.py
index 3f95282bd0f..678322f3b3f 100644
--- a/readthedocs/doc_builder/python_environments.py
+++ b/readthedocs/doc_builder/python_environments.py
@@ -425,9 +425,23 @@ def install_requirements_file(self, install):
             self.build_env.run(
                 *args,
                 cwd=self.checkout_path,
-                bin_path=self.venv_bin()  # noqa - no comma here in py27 :/
+                bin_path=self.venv_bin(),
             )
 
+    def list_packages_installed(self):
+        """List packages installed in pip."""
+        args = [
+            self.venv_bin(filename='python'),
+            '-m',
+            'pip',
+            'list',
+        ]
+        self.build_env.run(
+            *args,
+            cwd=self.checkout_path,
+            bin_path=self.venv_bin(),
+        )
+
 
 class Conda(PythonEnvironment):
 
@@ -627,3 +641,15 @@ def install_requirements_file(self, install):
         # as the conda environment was created by using the ``environment.yml``
         # defined by the user, there is nothing to update at this point
         pass
+
+    def list_packages_installed(self):
+        """List packages installed in conda."""
+        args = [
+            'conda',
+            'list',
+        ]
+        self.build_env.run(
+            *args,
+            cwd=self.checkout_path,
+            bin_path=self.venv_bin(),
+        )
diff --git a/readthedocs/projects/models.py b/readthedocs/projects/models.py
index a5f64eb4e7a..a776f77e105 100644
--- a/readthedocs/projects/models.py
+++ b/readthedocs/projects/models.py
@@ -1584,6 +1584,7 @@ def add_features(sender, **kwargs):
     CACHED_ENVIRONMENT = 'cached_environment'
     CELERY_ROUTER = 'celery_router'
     LIMIT_CONCURRENT_BUILDS = 'limit_concurrent_builds'
+    LIST_PACKAGES_INSTALLED_ENV = 'list_packages_installed_env'
     VCS_REMOTE_LISTING = 'vcs_remote_listing'
 
     FEATURES = (
@@ -1663,6 +1664,13 @@ def add_features(sender, **kwargs):
             LIMIT_CONCURRENT_BUILDS,
             _('Limit the amount of concurrent builds'),
         ),
+        (
+            LIST_PACKAGES_INSTALLED_ENV,
+            _(
+                'List packages installed in the environment ("pip list" or "conda list") '
+                'on build\'s output',
+            ),
+        ),
         (
             VCS_REMOTE_LISTING,
             _('Use remote listing in VCS (e.g. git ls-remote) if supported for sync versions'),
diff --git a/readthedocs/projects/tasks.py b/readthedocs/projects/tasks.py
index 668b00cf963..2b9b67ec147 100644
--- a/readthedocs/projects/tasks.py
+++ b/readthedocs/projects/tasks.py
@@ -1205,6 +1205,8 @@ def setup_python_environment(self):
         self.python_env.save_environment_json()
         self.python_env.install_core_requirements()
         self.python_env.install_requirements()
+        if self.project.has_feature(Feature.LIST_PACKAGES_INSTALLED_ENV):
+            self.python_env.list_packages_installed()
 
     def build_docs(self):
         """