diff --git a/spec/unit/room-upgrade.spec.ts b/spec/unit/room-upgrade.spec.ts
new file mode 100644
index 000000000..e15753521
--- /dev/null
+++ b/spec/unit/room-upgrade.spec.ts
@@ -0,0 +1,57 @@
+/*
+Copyright 2026 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+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.
+*/
+
+import { MatrixClient } from "../../src/client";
+
+describe("Room upgrades", function () {
+    it("Sends an HTTP request upgrading the room", () => {
+        // Given a client with a fake authedRequest method
+        const { client, authedRequest } = createClient();
+
+        // When we upgrade the room to version 12
+        client.upgradeRoom("!r1", "12");
+
+        // Then we make an HTTP request to the correct endpoint, with the
+        // version provided in the JSON.
+        expect(authedRequest).toHaveBeenCalledWith("POST", "/rooms/!r1/upgrade", undefined, { new_version: "12" });
+    });
+
+    it("Includes additional_creators if provided", () => {
+        // Given a client with a fake authedRequest method
+        const { client, authedRequest } = createClient();
+
+        // When we upgrade the room to version 13 and supply additionalCreators
+        client.upgradeRoom("!r1", "13", ["@u:s.co", "@v:a.b"]);
+
+        // Then we make an HTTP request to the correct endpoint, with the
+        // version and additional creators provided.
+        expect(authedRequest).toHaveBeenCalledWith("POST", "/rooms/!r1/upgrade", undefined, {
+            new_version: "13",
+            additional_creators: ["@u:s.co", "@v:a.b"],
+        });
+    });
+});
+
+///
+function createClient(): { client: MatrixClient; authedRequest: any } {
+    const authedRequest = vi.fn();
+    const client = new MatrixClient({
+        baseUrl: "https://my.home.server",
+        userId: "@u:s.co",
+    });
+    client.http.authedRequest = authedRequest;
+    return { client, authedRequest };
+}
diff --git a/src/client.ts b/src/client.ts
index cfb6e0276..262183646 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -6937,13 +6937,23 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
     /**
      * Upgrades a room to a new protocol version
      * @param newVersion - The target version to upgrade to
+     * @param additionalCreators - an optional list of user IDs of users who
+     *        should have the same permissions as the user performing the
+     *        upgrade
      * @returns Promise which resolves: Object with key 'replacement_room'
      * @returns Rejects: with an error response.
      */
-    public upgradeRoom(roomId: string, newVersion: string): Promise<{ replacement_room: string }> {
+    public upgradeRoom(
+        roomId: string,
+        newVersion: string,
+        additionalCreators?: string[],
+    ): Promise<{ replacement_room: string }> {
         // eslint-disable-line camelcase
         const path = utils.encodeUri("/rooms/$roomId/upgrade", { $roomId: roomId });
-        return this.http.authedRequest(Method.Post, path, undefined, { new_version: newVersion });
+        return this.http.authedRequest(Method.Post, path, undefined, {
+            new_version: newVersion,
+            additional_creators: additionalCreators,
+        });
     }
 
     /**