diff --git a/changelog.d/15621.misc b/changelog.d/15621.misc
new file mode 100644
index 000000000..5d060f4db
--- /dev/null
+++ b/changelog.d/15621.misc
@@ -0,0 +1 @@
+Update Mutual Rooms (MSC2666) implementation to match new proposal text.
\ No newline at end of file
diff --git a/synapse/rest/client/mutual_rooms.py b/synapse/rest/client/mutual_rooms.py
index 38ef4e459..c99445da3 100644
--- a/synapse/rest/client/mutual_rooms.py
+++ b/synapse/rest/client/mutual_rooms.py
@@ -12,13 +12,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import TYPE_CHECKING, Tuple
+from http import HTTPStatus
+from typing import TYPE_CHECKING, Dict, List, Tuple
 
 from synapse.api.errors import Codes, SynapseError
 from synapse.http.server import HttpServer
-from synapse.http.servlet import RestServlet
+from synapse.http.servlet import RestServlet, parse_strings_from_args
 from synapse.http.site import SynapseRequest
-from synapse.types import JsonDict, UserID
+from synapse.types import JsonDict
 
 from ._base import client_patterns
 
@@ -30,11 +31,11 @@ logger = logging.getLogger(__name__)
 
 class UserMutualRoomsServlet(RestServlet):
     """
-    GET /uk.half-shot.msc2666/user/mutual_rooms/{user_id} HTTP/1.1
+    GET /uk.half-shot.msc2666/user/mutual_rooms?user_id={user_id} HTTP/1.1
     """
 
     PATTERNS = client_patterns(
-        "/uk.half-shot.msc2666/user/mutual_rooms/(?P<user_id>[^/]*)",
+        "/uk.half-shot.msc2666/user/mutual_rooms$",
         releases=(),  # This is an unstable feature
     )
 
@@ -43,17 +44,35 @@ class UserMutualRoomsServlet(RestServlet):
         self.auth = hs.get_auth()
         self.store = hs.get_datastores().main
 
-    async def on_GET(
-        self, request: SynapseRequest, user_id: str
-    ) -> Tuple[int, JsonDict]:
-        UserID.from_string(user_id)
+    async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
+        # twisted.web.server.Request.args is incorrectly defined as Optional[Any]
+        args: Dict[bytes, List[bytes]] = request.args  # type: ignore
+
+        user_ids = parse_strings_from_args(args, "user_id", required=True)
+
+        if len(user_ids) > 1:
+            raise SynapseError(
+                HTTPStatus.BAD_REQUEST,
+                "Duplicate user_id query parameter",
+                errcode=Codes.INVALID_PARAM,
+            )
+
+        # We don't do batching, so a batch token is illegal by default
+        if b"batch_token" in args:
+            raise SynapseError(
+                HTTPStatus.BAD_REQUEST,
+                "Unknown batch_token",
+                errcode=Codes.INVALID_PARAM,
+            )
+
+        user_id = user_ids[0]
 
         requester = await self.auth.get_user_by_req(request)
         if user_id == requester.user.to_string():
             raise SynapseError(
-                code=400,
-                msg="You cannot request a list of shared rooms with yourself",
-                errcode=Codes.FORBIDDEN,
+                HTTPStatus.UNPROCESSABLE_ENTITY,
+                "You cannot request a list of shared rooms with yourself",
+                errcode=Codes.INVALID_PARAM,
             )
 
         rooms = await self.store.get_mutual_rooms_between_users(
diff --git a/synapse/rest/client/versions.py b/synapse/rest/client/versions.py
index 58c5b0739..32df054f5 100644
--- a/synapse/rest/client/versions.py
+++ b/synapse/rest/client/versions.py
@@ -91,7 +91,7 @@ class VersionsRestServlet(RestServlet):
                     # Implements additional endpoints as described in MSC2432
                     "org.matrix.msc2432": True,
                     # Implements additional endpoints as described in MSC2666
-                    "uk.half-shot.msc2666.mutual_rooms": True,
+                    "uk.half-shot.msc2666.query_mutual_rooms": True,
                     # Whether new rooms will be set to encrypted or not (based on presets).
                     "io.element.e2ee_forced.public": self.e2ee_forced_public,
                     "io.element.e2ee_forced.private": self.e2ee_forced_private,
diff --git a/tests/rest/client/test_mutual_rooms.py b/tests/rest/client/test_mutual_rooms.py
index a4327f7ac..22fddbd6d 100644
--- a/tests/rest/client/test_mutual_rooms.py
+++ b/tests/rest/client/test_mutual_rooms.py
@@ -11,6 +11,8 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from urllib.parse import quote
+
 from twisted.test.proto_helpers import MemoryReactor
 
 import synapse.rest.admin
@@ -44,8 +46,8 @@ class UserMutualRoomsTest(unittest.HomeserverTestCase):
     def _get_mutual_rooms(self, token: str, other_user: str) -> FakeChannel:
         return self.make_request(
             "GET",
-            "/_matrix/client/unstable/uk.half-shot.msc2666/user/mutual_rooms/%s"
-            % other_user,
+            "/_matrix/client/unstable/uk.half-shot.msc2666/user/mutual_rooms"
+            f"?user_id={quote(other_user)}",
             access_token=token,
         )