diff --git a/changelog.d/19554.feature b/changelog.d/19554.feature
new file mode 100644
index 00000000000..d30d3da1f6c
--- /dev/null
+++ b/changelog.d/19554.feature
@@ -0,0 +1 @@
+When Matrix Authentication Service (MAS) integration is enabled, allow MAS to set the user locked status in Synapse.
\ No newline at end of file
diff --git a/synapse/rest/synapse/mas/users.py b/synapse/rest/synapse/mas/users.py
index 55c73375551..01db41bcfac 100644
--- a/synapse/rest/synapse/mas/users.py
+++ b/synapse/rest/synapse/mas/users.py
@@ -112,6 +112,18 @@ class PostBody(RequestBodyModel):
         unset_emails: StrictBool = False
         set_emails: list[StrictStr] | None = None
 
+        locked: StrictBool | None = None
+        """
+        True to lock user. False to unlock. None to leave the same.
+
+        This is mostly for informational purposes; if the user's account is locked in MAS
+        but not in Synapse, the token introspection response will prevent them from using
+        their account.
+
+        However, having a local copy of the locked state in Synapse is useful for excluding
+        the user from the user directory.
+        """
+
         @model_validator(mode="before")
         @classmethod
         def validate_exclusive(cls, values: Any) -> Any:
@@ -206,6 +218,9 @@ async def _async_render_POST(
                         validated_at=current_time,
                     )
 
+        if body.locked is not None:
+            await self.store.set_user_locked_status(user_id.to_string(), body.locked)
+
         if body.unset_avatar_url:
             await self.profile_handler.set_avatar_url(
                 target_user=user_id,
diff --git a/tests/rest/synapse/mas/test_users.py b/tests/rest/synapse/mas/test_users.py
index f0f26a939c9..6f44761bb8b 100644
--- a/tests/rest/synapse/mas/test_users.py
+++ b/tests/rest/synapse/mas/test_users.py
@@ -362,6 +362,64 @@ def test_provision_user_invalid_json_types(self) -> None:
             )
             self.assertEqual(channel.code, 400, f"Should fail for content: {content}")
 
+    def test_lock_and_unlock(self) -> None:
+        store = self.hs.get_datastores().main
+
+        # Create a user in the locked state
+        alice = UserID("alice", "test")
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/provision_user",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": alice.localpart,
+                "locked": True,
+            },
+        )
+        # This created the user, hence the 201 status code
+        self.assertEqual(channel.code, 201, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+        self.assertTrue(
+            self.get_success(store.get_user_locked_status(alice.to_string()))
+        )
+
+        # Then transition from locked to unlocked
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/provision_user",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": alice.localpart,
+                "locked": False,
+            },
+        )
+        # This updated the user, hence the 200 status code
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+        self.assertFalse(
+            self.get_success(store.get_user_locked_status(alice.to_string()))
+        )
+
+        # And back from unlocked to locked
+        channel = self.make_request(
+            "POST",
+            "/_synapse/mas/provision_user",
+            shorthand=False,
+            access_token=self.SHARED_SECRET,
+            content={
+                "localpart": alice.localpart,
+                "locked": True,
+            },
+        )
+        # This updated the user, hence the 200 status code
+        self.assertEqual(channel.code, 200, channel.json_body)
+        self.assertEqual(channel.json_body, {})
+        self.assertTrue(
+            self.get_success(store.get_user_locked_status(alice.to_string()))
+        )
+
 
 @skip_unless(HAS_AUTHLIB, "requires authlib")
 class MasIsLocalpartAvailableResource(BaseTestCase):