diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index c0ae9e5d7aae68..5afc0dc556dfea 100644
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -632,6 +632,12 @@ functions.
    the value in ``pw_uid`` will be used. If the value is an integer, it will
    be passed verbatim. (POSIX only)
 
+   .. note::
+
+      Specifying *user* will not drop existing supplementary group memberships!
+      The caller must also pass ``extra_groups=()`` to reduce the group membership
+      of the child process for security purposes.
+
    .. availability:: POSIX
    .. versionadded:: 3.9