diff --git a/tox.ini b/tox.ini
index b6cb4bc54..3a58cc0cf 100644
--- a/tox.ini
+++ b/tox.ini
@@ -105,6 +105,7 @@ extension =
    N340 = checks:check_oslo_i18n_wrapper
    N341 = checks:check_builtins_gettext
    N342 = checks:no_redundant_import_alias
+   N366 = checks:import_stock_mock
 paths = ./watcher/hacking
 
 
diff --git a/watcher/hacking/checks.py b/watcher/hacking/checks.py
index c4bfb85f3..f0e9a51b0 100644
--- a/watcher/hacking/checks.py
+++ b/watcher/hacking/checks.py
@@ -285,3 +285,31 @@ def no_redundant_import_alias(logical_line):
     """
     if re.match(re_redundant_import_alias, logical_line):
         yield(0, "N342: No redundant import alias.")
+
+
+@flake8ext
+def import_stock_mock(logical_line):
+    """Use python's mock, not the mock library.
+
+    Since we `dropped support for python 2`__, we no longer need to use the
+    mock library, which existed to backport py3 functionality into py2.
+    Which must be done by saying::
+
+        from unittest import mock
+
+    ...because if you say::
+
+        import mock
+
+    ...you definitely will not be getting the standard library mock. That will
+    always import the third party mock library. This check can be removed in
+    the future (and we can start saying ``import mock`` again) if we manage to
+    purge these transitive dependencies.
+
+    .. __: https://review.opendev.org/#/c/717540
+
+    N366
+    """
+    if logical_line == 'import mock':
+        yield (0, "N366: You must explicitly import python's mock: "
+                  "``from unittest import mock``")