diff --git a/changelog.d/15227.bugfix b/changelog.d/15227.bugfix
new file mode 100644
index 000000000000..eaa26c8f7fa3
--- /dev/null
+++ b/changelog.d/15227.bugfix
@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.79.0rc1 where attempting to register a `on_remove_user_third_party_identifier` module API callback would be a no-op.
\ No newline at end of file
diff --git a/synapse/events/third_party_rules.py b/synapse/events/third_party_rules.py
index 3e4d52c8d803..61d4530be784 100644
--- a/synapse/events/third_party_rules.py
+++ b/synapse/events/third_party_rules.py
@@ -247,6 +247,11 @@ def register_third_party_rules_callbacks(
                 on_add_user_third_party_identifier
             )
 
+        if on_remove_user_third_party_identifier is not None:
+            self._on_remove_user_third_party_identifier_callbacks.append(
+                on_remove_user_third_party_identifier
+            )
+
     async def check_event_allowed(
         self,
         event: EventBase,
diff --git a/tests/rest/client/test_third_party_rules.py b/tests/rest/client/test_third_party_rules.py
index 3b9951370752..753ecc8d161a 100644
--- a/tests/rest/client/test_third_party_rules.py
+++ b/tests/rest/client/test_third_party_rules.py
@@ -941,18 +941,16 @@ def test_on_add_and_remove_user_third_party_identifier(self) -> None:
         just before associating and removing a 3PID to/from an account.
         """
         # Pretend to be a Synapse module and register both callbacks as mocks.
-        third_party_rules = self.hs.get_third_party_event_rules()
         on_add_user_third_party_identifier_callback_mock = Mock(
             return_value=make_awaitable(None)
         )
         on_remove_user_third_party_identifier_callback_mock = Mock(
             return_value=make_awaitable(None)
         )
-        third_party_rules._on_threepid_bind_callbacks.append(
-            on_add_user_third_party_identifier_callback_mock
-        )
-        third_party_rules._on_threepid_bind_callbacks.append(
-            on_remove_user_third_party_identifier_callback_mock
+        third_party_rules = self.hs.get_third_party_event_rules()
+        third_party_rules.register_third_party_rules_callbacks(
+            on_add_user_third_party_identifier=on_add_user_third_party_identifier_callback_mock,
+            on_remove_user_third_party_identifier=on_remove_user_third_party_identifier_callback_mock,
         )
 
         # Register an admin user.
@@ -1008,12 +1006,12 @@ def test_on_remove_user_third_party_identifier_is_called_on_deactivate(
         when a user is deactivated and their third-party ID associations are deleted.
         """
         # Pretend to be a Synapse module and register both callbacks as mocks.
-        third_party_rules = self.hs.get_third_party_event_rules()
         on_remove_user_third_party_identifier_callback_mock = Mock(
             return_value=make_awaitable(None)
         )
-        third_party_rules._on_threepid_bind_callbacks.append(
-            on_remove_user_third_party_identifier_callback_mock
+        third_party_rules = self.hs.get_third_party_event_rules()
+        third_party_rules.register_third_party_rules_callbacks(
+            on_remove_user_third_party_identifier=on_remove_user_third_party_identifier_callback_mock,
         )
 
         # Register an admin user.
@@ -1039,6 +1037,9 @@ def test_on_remove_user_third_party_identifier_is_called_on_deactivate(
         )
         self.assertEqual(channel.code, 200, channel.json_body)
 
+        # Check that the mock was not called on the act of adding a third-party ID.
+        on_remove_user_third_party_identifier_callback_mock.assert_not_called()
+
         # Now deactivate the user.
         channel = self.make_request(
             "PUT",