diff --git a/changelog.d/20164.misc b/changelog.d/20164.misc
new file mode 100644
index 00000000000..a322e1d9d1f
--- /dev/null
+++ b/changelog.d/20164.misc
@@ -0,0 +1 @@
+Add unit tests for `is_compliant_user_id_localpart`. Contributed by @guillemo12.
diff --git a/tests/test_types.py b/tests/test_types.py
index 43fd96d6f55..a19db1804b1 100644
--- a/tests/test_types.py
+++ b/tests/test_types.py
@@ -37,6 +37,7 @@
     UserID,
     get_domain_from_id,
     get_localpart_from_id,
+    is_compliant_user_id_localpart,
     map_username_to_mxid_localpart,
 )
 
@@ -350,3 +351,27 @@ class MyModel(BaseModel):
 
         self.assertEqual(MyModel.model_validate_json('{"limit": 0}'), MyModel(limit=0))
         self.assertEqual(MyModel.model_validate({"limit": 42}), MyModel(limit=42))
+
+
+class IsCompliantUserIdLocalpartTestCase(unittest.TestCase):
+    def test_empty_string(self) -> None:
+        self.assertFalse(is_compliant_user_id_localpart(""))
+
+    def test_valid_characters(self) -> None:
+        # alphanumeric
+        self.assertTrue(is_compliant_user_id_localpart("alice"))
+        self.assertTrue(is_compliant_user_id_localpart("Alice123"))
+
+        # symbols between 0x21 and 0x7E
+        self.assertTrue(is_compliant_user_id_localpart("!@#$"))
+        self.assertTrue(is_compliant_user_id_localpart("bob.jones"))
+        self.assertTrue(is_compliant_user_id_localpart("=+-[]{}"))
+
+    def test_invalid_characters(self) -> None:
+        # non-ascii / outside range
+        self.assertFalse(is_compliant_user_id_localpart("álice"))
+        self.assertFalse(is_compliant_user_id_localpart("alice 123"))  # space is 0x20
+        self.assertFalse(is_compliant_user_id_localpart("alice\t"))
+        self.assertFalse(is_compliant_user_id_localpart("alice\n"))
+        self.assertFalse(is_compliant_user_id_localpart("alice\x1f"))  # control char
+        self.assertFalse(is_compliant_user_id_localpart("alice\x7f"))  # DEL is 0x7F