diff --git a/src/commands/CommandHandler.ts b/src/commands/CommandHandler.ts
index c306480b..0b778747 100644
--- a/src/commands/CommandHandler.ts
+++ b/src/commands/CommandHandler.ts
@@ -53,6 +53,7 @@ import { execUnsuspendCommand } from "./UnsuspendCommand";
 import { execIgnoreCommand, execListIgnoredCommand } from "./IgnoreCommand";
 import { execLockCommand } from "./LockCommand";
 import { execUnlockCommand } from "./UnlockCommand";
+import { execLockdownCommand } from "./LockdownCommand";
 
 export const COMMAND_PREFIX = "!mjolnir";
 
@@ -152,6 +153,8 @@ export async function handleCommand(roomId: string, event: { content: { body: st
             return await execLockCommand(roomId, event, mjolnir, parts);
         } else if (parts[1] === "unlock") {
             return await execUnlockCommand(roomId, event, mjolnir, parts);
+        } else if (parts[1] === "lockdown") {
+            return await execLockdownCommand(roomId, event, mjolnir, parts);
         } else if (parts[1] === "help") {
             // Help menu
             const protectionMenu =
@@ -182,6 +185,7 @@ export async function handleCommand(roomId: string, event: { content: { body: st
                 "!mjolnir unlock <user ID>                                              - Unlock the account of the specified user\n" +
                 "!mjolnir ignore <user ID/server name>                                 - Add user to list of users/servers that cannot be banned/ACL'd. Note that this does not survive restart.\n" +
                 "!mjolnir ignored                                                      - List currently ignored entities.\n" +
+                "!mjolnir lockdown (lock|unlock) [room alias/ID]                                - Locks a room to invite-only. If not specified, this applies to all protected rooms.\n" +
                 "!mjolnir shutdown room <room alias/ID> [message]                      - Uses the bot's account to shut down a room, preventing access to the room on this server\n";
 
             const policyListMenu =
diff --git a/src/commands/LockdownCommand.ts b/src/commands/LockdownCommand.ts
new file mode 100644
index 00000000..3d2ff209
--- /dev/null
+++ b/src/commands/LockdownCommand.ts
@@ -0,0 +1,95 @@
+/*
+Copyright 2025 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 { MatrixSendClient } from "../MatrixEmitter";
+import { Mjolnir } from "../Mjolnir";
+import { JoinRulesEventContent, LogLevel, LogService } from "@vector-im/matrix-bot-sdk";
+
+export const LOCKDOWN_EVENT_TYPE = "org.matrix.mjolnir.lockdown";
+
+// !mjolnir lockdown [roomId]
+export async function execLockdownCommand(managementRoomId: string, event: any, mjolnir: Mjolnir, parts: string[]) {
+    const lockOrUnlock = parts[2]?.toLocaleLowerCase();
+    const target = parts[3];
+
+    if (!["lock", "unlock"].includes(lockOrUnlock)) {
+        throw Error("Command must be lock or unlock");
+    }
+
+    let targetRooms: string[];
+    if (target) {
+        const targetRoomId = await mjolnir.client.resolveRoom(target);
+        targetRooms = [targetRoomId];
+    } else if (mjolnir.config.protectAllJoinedRooms) {
+        targetRooms = await mjolnir.client.getJoinedRooms();
+    } else {
+        targetRooms = mjolnir.protectedRoomsConfig.getExplicitlyProtectedRooms();
+    }
+
+    if (!targetRooms.length) {
+        await mjolnir.managementRoomOutput.logMessage(LogLevel.INFO, "LockdownCommand", "No protected rooms found");
+        return;
+    }
+    await mjolnir.managementRoomOutput.logMessage(
+        LogLevel.INFO,
+        "LockdownCommand",
+        target ? `Locking down room` : "Locking down ALL protected rooms",
+    );
+    await mjolnir.client.unstableApis.addReactionToEvent(managementRoomId, event["event_id"], "⏳");
+    let didError = false;
+    for (const roomId of targetRooms) {
+        try {
+            await ensureLockdownState(mjolnir.client, roomId, lockOrUnlock === "lock");
+        } catch (ex) {
+            mjolnir.managementRoomOutput.logMessage(
+                LogLevel.ERROR,
+                "Lock Command",
+                `There was an error locking ${target}, please check the logs for more information.`,
+            );
+            LogService.error("LockdownCommand", `Error changing lockdown state of ${roomId}:`, ex);
+            didError = true;
+            await mjolnir.client.unstableApis.addReactionToEvent(roomId, event["event_id"], "❌");
+        }
+    }
+
+    if (!didError) {
+        await mjolnir.client.unstableApis.addReactionToEvent(managementRoomId, event["event_id"], "✅");
+    }
+}
+
+async function ensureLockdownState(client: MatrixSendClient, roomId: string, lockdown: boolean) {
+    const currentState = await client.getSafeRoomAccountData<
+        { locked: false } | { locked: true; previousState: JoinRulesEventContent }
+    >(LOCKDOWN_EVENT_TYPE, roomId, { locked: false });
+    const currentJoinRule = (await client.getRoomStateEvent(roomId, "m.room.join_rules", "")) as JoinRulesEventContent;
+    if (!currentState.locked && lockdown) {
+        const newState = {
+            locked: true,
+            previousState: currentJoinRule,
+        };
+        await client.sendStateEvent(roomId, "m.room.join_rules", "", {
+            join_rule: "invite",
+        });
+        await client.setRoomAccountData(LOCKDOWN_EVENT_TYPE, roomId, newState);
+    } else if (currentState.locked && !lockdown) {
+        const newState = {
+            locked: false,
+        };
+        await client.sendStateEvent(roomId, "m.room.join_rules", "", currentState.previousState);
+        await client.setRoomAccountData(LOCKDOWN_EVENT_TYPE, roomId, newState);
+    }
+    // Else, nothing to do.
+}
diff --git a/test/integration/commands/lockdownCommandTest.ts b/test/integration/commands/lockdownCommandTest.ts
new file mode 100644
index 00000000..a2815d55
--- /dev/null
+++ b/test/integration/commands/lockdownCommandTest.ts
@@ -0,0 +1,43 @@
+import { strict as assert } from "assert";
+
+import { newTestUser } from "../clientHelper";
+import { getFirstReaction } from "./commandUtils";
+import { MatrixClient } from "@vector-im/matrix-bot-sdk";
+
+describe("Test: lockdown command", function () {
+    let client: MatrixClient;
+    this.beforeEach(async function () {
+        client = await newTestUser(this.config.homeserverUrl, { name: { contains: "lockdown-command" } });
+        await client.start();
+    });
+    this.afterEach(async function () {
+        await client.stop();
+    });
+    it("should lockdown a room", async function () {
+        this.timeout(20000);
+        const badRoom = await client.createRoom({ preset: "public_chat" });
+        await client.joinRoom(this.mjolnir.managementRoomId);
+
+        const reply1 = new Promise(async (resolve, reject) => {
+            const msgid = await client.sendMessage(this.mjolnir.managementRoomId, {
+                msgtype: "m.text",
+                body: `!mjolnir lockdown lock ${badRoom}`,
+            });
+            client.on("room.event", (roomId, event) => {
+                if (
+                    roomId === this.mjolnir.managementRoomId &&
+                    event?.type === "m.reaction" &&
+                    event.sender === this.mjolnir.client.userId &&
+                    event.content?.["m.relates_to"]?.event_id === msgid
+                ) {
+                    resolve(event);
+                }
+            });
+        });
+
+        await reply1;
+
+        const newJoinRules = await client.getRoomStateEvent(badRoom, "m.room.join_rules", "");
+        console.log(newJoinRules);
+    });
+});