diff --git a/src/app/features/room/schedule-send/ScheduledMessagesList.test.tsx b/src/app/features/room/schedule-send/ScheduledMessagesList.test.tsx
new file mode 100644
index 000000000..2137f9248
--- /dev/null
+++ b/src/app/features/room/schedule-send/ScheduledMessagesList.test.tsx
@@ -0,0 +1,197 @@
+/* oxlint-disable typescript/no-explicit-any, typescript/no-extraneous-class, unicorn/consistent-function-scoping, vitest/require-mock-type-parameters */
+
+import { fireEvent, render, screen, waitFor } from '@testing-library/react';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { ScheduledMessagesList } from './ScheduledMessagesList';
+import type * as MatrixSdkModule from '$types/matrix-sdk';
+
+const testState = vi.hoisted(() => ({
+  cancelDelayedEvent: vi.fn(),
+  coordinatorRun: vi.fn(),
+  invalidateQueries: vi.fn(),
+  matrix: {
+    getSafeUserId: vi.fn(() => '@me:example.org'),
+  },
+}));
+
+vi.mock('$hooks/useMatrixClient', () => ({
+  useMatrixClient: () => testState.matrix,
+}));
+
+vi.mock('$utils/delayedEvents', () => ({
+  cancelDelayedEvent: testState.cancelDelayedEvent,
+  getDelayedEvents: vi.fn(),
+}));
+
+vi.mock('$state/room/roomScheduleCoordinator', () => ({
+  roomScheduleCoordinator: {
+    run: testState.coordinatorRun,
+  },
+}));
+
+vi.mock('@tanstack/react-query', () => ({
+  useQuery: () => ({
+    data: {
+      delayed_events: [
+        {
+          delay_id: 'delay-1',
+          room_id: '!room:example.org',
+          type: 'm.room.message',
+          content: { body: 'Scheduled message' },
+          running_since: 1_000,
+          delay: 60_000,
+        },
+      ],
+    },
+  }),
+  useQueryClient: () => ({ invalidateQueries: testState.invalidateQueries }),
+}));
+
+vi.mock('$state/scheduledMessages', async () => {
+  const { atom } = await import('jotai');
+  const scheduledTimeAtom = atom<Date | null>(null);
+  const editingScheduledDelayIdAtom = atom<string | null>(null);
+
+  return {
+    delayedEventsSupportedAtom: atom(true),
+    roomIdToScheduledTimeAtomFamily: () => scheduledTimeAtom,
+    roomIdToEditingScheduledDelayIdAtomFamily: () => editingScheduledDelayIdAtom,
+  };
+});
+
+vi.mock('$state/hooks/settings', () => ({
+  useSetting: (_atom: unknown, key: string) => [
+    key === 'hour24Clock' ? false : 'YYYY-MM-DD',
+    vi.fn(),
+  ],
+}));
+
+vi.mock('$state/settings', () => ({ settingsAtom: {} }));
+
+vi.mock('$types/matrix-sdk', async (importOriginal) => ({
+  ...(await importOriginal<typeof MatrixSdkModule>()),
+  MatrixEvent: class MatrixEvent {},
+}));
+
+vi.mock('$utils/time', () => ({
+  timeDayMonthYear: () => '2026-07-28',
+  timeHourMinute: () => '00:01',
+}));
+
+vi.mock('$components/message-preview', () => ({
+  MessagePreview: ({ actions, event }: any) => (
+    <div>
+      <span>{event.getContent?.().body ?? 'Scheduled message'}</span>
+      {actions}
+    </div>
+  ),
+  useRoomMessagePreviewRenderer: () => vi.fn(),
+}));
+
+vi.mock('$components/icons/phosphor', () => ({
+  CaretDown: 'CaretDown',
+  CaretUp: 'CaretUp',
+  Clock: 'Clock',
+  Lock: 'Lock',
+  PencilSimple: 'PencilSimple',
+  X: 'X',
+  chipIcon: () => null,
+}));
+
+vi.mock('folds', () => {
+  const Box = ({ children, ...props }: any) => <div {...props}>{children}</div>;
+  const Text = ({ children, ...props }: any) => <span {...props}>{children}</span>;
+  const Button = ({ children, ...props }: any) => <button {...props}>{children}</button>;
+
+  return {
+    Box,
+    Chip: Button,
+    IconButton: Button,
+    Spinner: () => <span role="progressbar">Cancelling</span>,
+    Text,
+    config: {
+      borderWidth: { B300: '1px' },
+      space: { S100: '1px', S200: '2px', S400: '4px' },
+    },
+    toRem: (value: number) => `${value / 16}rem`,
+  };
+});
+
+vi.mock('./SchedulePickerDialog', () => ({
+  SchedulePickerDialog: () => null,
+}));
+
+const room = { roomId: '!room:example.org' } as any;
+
+function deferred<T>() {
+  let resolve!: (value: T) => void;
+  let reject!: (reason?: unknown) => void;
+  const promise = new Promise<T>((promiseResolve, promiseReject) => {
+    resolve = promiseResolve;
+    reject = promiseReject;
+  });
+  return { promise, resolve, reject };
+}
+
+function renderExpandedList() {
+  render(<ScheduledMessagesList room={room} />);
+  fireEvent.click(screen.getByRole('button', { name: /1 scheduled message/i }));
+  return screen.getByRole('button', { name: 'Cancel scheduled message' });
+}
+
+describe('ScheduledMessagesList cancellation', () => {
+  beforeEach(() => {
+    testState.cancelDelayedEvent.mockReset();
+    testState.coordinatorRun.mockReset();
+    testState.invalidateQueries.mockReset();
+    testState.coordinatorRun.mockImplementation((_roomId: string, operation: () => unknown) =>
+      operation()
+    );
+  });
+
+  it('blocks duplicate cancellation clicks and shows pending state', () => {
+    const pending = deferred<void>();
+    testState.cancelDelayedEvent.mockReturnValue(pending.promise);
+
+    const cancel = renderExpandedList();
+    fireEvent.click(cancel);
+    fireEvent.click(cancel);
+
+    expect(testState.coordinatorRun).toHaveBeenCalledWith(room.roomId, expect.any(Function));
+    expect(testState.cancelDelayedEvent).toHaveBeenCalledOnce();
+    expect(cancel).toBeDisabled();
+    expect(cancel).toHaveAttribute('aria-busy', 'true');
+    expect(screen.getByRole('progressbar')).toBeInTheDocument();
+  });
+
+  it('shows a retryable alert when cancellation fails', async () => {
+    testState.cancelDelayedEvent.mockRejectedValueOnce(new Error('Cancel failed'));
+    const cancel = renderExpandedList();
+
+    fireEvent.click(cancel);
+
+    await waitFor(() =>
+      expect(screen.getByRole('alert')).toHaveTextContent(
+        'Failed to cancel scheduled message. Try again.'
+      )
+    );
+    expect(cancel).not.toBeDisabled();
+    expect(cancel).not.toHaveAttribute('aria-busy', 'true');
+  });
+
+  it('successfully retries cancellation after a failure', async () => {
+    testState.cancelDelayedEvent
+      .mockRejectedValueOnce(new Error('Cancel failed'))
+      .mockResolvedValueOnce(undefined);
+    const cancel = renderExpandedList();
+
+    fireEvent.click(cancel);
+    await waitFor(() => expect(screen.getByRole('alert')).toBeInTheDocument());
+
+    fireEvent.click(cancel);
+
+    await waitFor(() => expect(testState.cancelDelayedEvent).toHaveBeenCalledTimes(2));
+    await waitFor(() => expect(testState.invalidateQueries).toHaveBeenCalledOnce());
+    expect(screen.queryByRole('alert')).not.toBeInTheDocument();
+  });
+});
diff --git a/src/app/features/room/schedule-send/ScheduledMessagesList.tsx b/src/app/features/room/schedule-send/ScheduledMessagesList.tsx
index 16583b392..c9aafa05a 100644
--- a/src/app/features/room/schedule-send/ScheduledMessagesList.tsx
+++ b/src/app/features/room/schedule-send/ScheduledMessagesList.tsx
@@ -1,6 +1,6 @@
-import { useCallback, useMemo, useState } from 'react';
+import { useCallback, useMemo, useRef, useState } from 'react';
 import { useQuery, useQueryClient } from '@tanstack/react-query';
-import { Box, Text, Chip, IconButton } from 'folds';
+import { Box, Text, Chip, IconButton, Spinner } from 'folds';
 import {
   CaretDown,
   CaretUp,
@@ -27,6 +27,7 @@ import {
 import { timeHourMinute, timeDayMonthYear } from '$utils/time';
 import { useSetting } from '$state/hooks/settings';
 import { settingsAtom } from '$state/settings';
+import { roomScheduleCoordinator } from '$state/room/roomScheduleCoordinator';
 import { MessagePreview, useRoomMessagePreviewRenderer } from '$components/message-preview';
 import { SchedulePickerDialog } from './SchedulePickerDialog';
 import * as css from './ScheduledMessagesList.css';
@@ -44,6 +45,12 @@ type ScheduledMessageRowProps = {
   hour24Clock: boolean;
   onEdit: (delayId: string, body: string, formattedBody?: string, scheduledTs?: number) => void;
   onCancel: (delayId: string) => void;
+  cancellationState: CancellationState;
+};
+
+type CancellationState = {
+  status: 'idle' | 'pending' | 'error';
+  error?: string;
 };
 
 function ScheduledMessageRow({
@@ -52,6 +59,7 @@ function ScheduledMessageRow({
   hour24Clock,
   onEdit,
   onCancel,
+  cancellationState,
 }: ScheduledMessageRowProps) {
   const mx = useMatrixClient();
   const [dateFormatString] = useSetting(settingsAtom, 'dateFormatString');
@@ -63,6 +71,8 @@ function ScheduledMessageRow({
     !isEncrypted && typeof event.content.formatted_body === 'string'
       ? event.content.formatted_body
       : undefined;
+  const isCancelling = cancellationState.status === 'pending';
+  const cancelIcon = isCancelling ? <Spinner size="100" /> : chipIcon(X);
   const matrixEvent = useMemo(
     () =>
       new MatrixEvent({
@@ -106,9 +116,11 @@ function ScheduledMessageRow({
                 variant="Critical"
                 radii="300"
                 onClick={() => onCancel(event.delay_id)}
+                disabled={isCancelling}
+                aria-busy={isCancelling}
                 aria-label="Cancel scheduled message"
               >
-                {chipIcon(X)}
+                {cancelIcon}
               </IconButton>
             </Box>
           }
@@ -126,12 +138,19 @@ function ScheduledMessageRow({
             variant="Critical"
             radii="300"
             onClick={() => onCancel(event.delay_id)}
+            disabled={isCancelling}
+            aria-busy={isCancelling}
             aria-label="Cancel scheduled message"
           >
-            {chipIcon(X)}
+            {cancelIcon}
           </IconButton>
         )}
       </Box>
+      {cancellationState.status === 'error' && (
+        <Text size="T200" priority="300" role="alert" aria-live="polite">
+          {cancellationState.error}
+        </Text>
+      )}
     </Box>
   );
 }
@@ -146,6 +165,10 @@ export function ScheduledMessagesList({ room, onEditMessage }: ScheduledMessages
   const [editingDelayId, setEditingDelayId] = useAtom(
     roomIdToEditingScheduledDelayIdAtomFamily(room.roomId)
   );
+  const [cancellationStates, setCancellationStates] = useState<Record<string, CancellationState>>(
+    {}
+  );
+  const pendingCancellations = useRef(new Set<string>());
 
   const { data } = useQuery({
     queryKey: ['delayedEvents', room.roomId],
@@ -167,10 +190,35 @@ export function ScheduledMessagesList({ room, onEditMessage }: ScheduledMessages
 
   const handleCancel = useCallback(
     async (delayId: string) => {
-      await cancelDelayedEvent(mx, delayId);
-      invalidateEvents();
+      if (pendingCancellations.current.has(delayId)) return;
+
+      pendingCancellations.current.add(delayId);
+      setCancellationStates((states) => ({
+        ...states,
+        [delayId]: { status: 'pending' },
+      }));
+
+      try {
+        await roomScheduleCoordinator.run(room.roomId, () => cancelDelayedEvent(mx, delayId));
+        invalidateEvents();
+        setCancellationStates((states) => {
+          const next = { ...states };
+          delete next[delayId];
+          return next;
+        });
+      } catch {
+        setCancellationStates((states) => ({
+          ...states,
+          [delayId]: {
+            status: 'error',
+            error: 'Failed to cancel scheduled message. Try again.',
+          },
+        }));
+      } finally {
+        pendingCancellations.current.delete(delayId);
+      }
     },
-    [mx, invalidateEvents]
+    [mx, room.roomId, invalidateEvents]
   );
 
   const handleEdit = useCallback(
@@ -218,6 +266,7 @@ export function ScheduledMessagesList({ room, onEditMessage }: ScheduledMessages
               hour24Clock={hour24Clock}
               onEdit={handleEdit}
               onCancel={handleCancel}
+              cancellationState={cancellationStates[event.delay_id] ?? { status: 'idle' }}
             />
           ))}
         </Box>
diff --git a/src/app/state/room/roomScheduleCoordinator.test.ts b/src/app/state/room/roomScheduleCoordinator.test.ts
new file mode 100644
index 000000000..98f67e548
--- /dev/null
+++ b/src/app/state/room/roomScheduleCoordinator.test.ts
@@ -0,0 +1,62 @@
+import { describe, expect, it, vi } from 'vitest';
+import { roomScheduleCoordinator } from './roomScheduleCoordinator';
+
+function deferred() {
+  let resolve!: () => void;
+  const promise = new Promise<void>((resolvePromise) => {
+    resolve = resolvePromise;
+  });
+  return { promise, resolve };
+}
+
+describe('roomScheduleCoordinator', () => {
+  it('runs operations in FIFO order for a room', async () => {
+    const first = deferred();
+    const order: string[] = [];
+
+    const firstOperation = roomScheduleCoordinator.run('!room:example.org', async () => {
+      order.push('first-start');
+      await first.promise;
+      order.push('first-end');
+    });
+    const secondOperation = roomScheduleCoordinator.run('!room:example.org', async () => {
+      order.push('second');
+    });
+
+    await new Promise<void>((resolve) => setTimeout(resolve, 0));
+    expect(order).toEqual(['first-start']);
+
+    first.resolve();
+    await Promise.all([firstOperation, secondOperation]);
+    expect(order).toEqual(['first-start', 'first-end', 'second']);
+  });
+
+  it('continues the room queue after an operation fails', async () => {
+    const second = vi.fn<() => Promise<string>>(async () => 'completed');
+
+    await expect(
+      roomScheduleCoordinator.run('!room:example.org', async () => {
+        throw new Error('cancel failed');
+      })
+    ).rejects.toThrow('cancel failed');
+    await expect(roomScheduleCoordinator.run('!room:example.org', second)).resolves.toBe(
+      'completed'
+    );
+
+    expect(second).toHaveBeenCalledOnce();
+  });
+
+  it('does not serialize operations for different rooms', async () => {
+    const first = deferred();
+    const second = vi.fn<() => Promise<string>>(async () => 'completed');
+
+    const firstOperation = roomScheduleCoordinator.run('!first:example.org', () => first.promise);
+    const secondOperation = roomScheduleCoordinator.run('!second:example.org', second);
+
+    await expect(secondOperation).resolves.toBe('completed');
+    expect(second).toHaveBeenCalledOnce();
+
+    first.resolve();
+    await firstOperation;
+  });
+});
diff --git a/src/app/state/room/roomScheduleCoordinator.ts b/src/app/state/room/roomScheduleCoordinator.ts
new file mode 100644
index 000000000..ddfa595ed
--- /dev/null
+++ b/src/app/state/room/roomScheduleCoordinator.ts
@@ -0,0 +1,4 @@
+import { createKeyedQueue } from '$utils/keyedQueue';
+
+/** Serializes delayed-event mutations for each room without blocking other rooms. */
+export const roomScheduleCoordinator = { run: createKeyedQueue() };