diff --git a/changelog.d/7751.misc b/changelog.d/7751.misc
new file mode 100644
index 000000000..eb10ecd92
--- /dev/null
+++ b/changelog.d/7751.misc
@@ -0,0 +1 @@
+Explain the "test" conditional requirement for dependencies is not all of the modules necessary to run the unit tests.
diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py
index 92d3709ae..b1cac901e 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -93,6 +93,10 @@ CONDITIONAL_REQUIREMENTS = {
     "oidc": ["authlib>=0.14.0"],
     "systemd": ["systemd-python>=231"],
     "url_preview": ["lxml>=3.5.0"],
+    # Dependencies which are exclusively required by unit test code. This is
+    # NOT a list of all modules that are necessary to run the unit tests.
+    # Tests assume that all optional dependencies are installed.
+    #
     # parameterized_class decorator was introduced in parameterized 0.7.0
     "test": ["mock>=2.0", "parameterized>=0.7.0"],
     "sentry": ["sentry-sdk>=0.7.2"],