diff --git a/CHANGELOG.md b/CHANGELOG.md
index e5bbda36f..c92b8c06a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -45,6 +45,7 @@
   (Cherry-picked from https://activitypub.software/TransFem-org/Sharkey/-/merge_requests/712)
 - Fix: FTT無効時にユーザーリストタイムラインが使用できない問題を修正  
   (Cherry-picked from https://activitypub.software/TransFem-org/Sharkey/-/merge_requests/709)
+- Fix: User Webhookテスト機能のMock Payloadを修正  
 
 ### Misskey.js
 - Fix: Stream初期化時、別途WebSocketを指定する場合の型定義を修正
diff --git a/packages/backend/src/core/QueueService.ts b/packages/backend/src/core/QueueService.ts
index 37028026c..50f08da24 100644
--- a/packages/backend/src/core/QueueService.ts
+++ b/packages/backend/src/core/QueueService.ts
@@ -7,7 +7,7 @@ import { randomUUID } from 'node:crypto';
 import { Inject, Injectable } from '@nestjs/common';
 import type { IActivity } from '@/core/activitypub/type.js';
 import type { MiDriveFile } from '@/models/DriveFile.js';
-import type { MiWebhook, webhookEventTypes } from '@/models/Webhook.js';
+import type { MiWebhook, WebhookEventTypes, webhookEventTypes } from '@/models/Webhook.js';
 import type { MiSystemWebhook, SystemWebhookEventType } from '@/models/SystemWebhook.js';
 import type { Config } from '@/config.js';
 import { DI } from '@/di-symbols.js';
@@ -35,6 +35,7 @@ import type {
 } from './QueueModule.js';
 import type httpSignature from '@peertube/http-signature';
 import type * as Bull from 'bullmq';
+import { type UserWebhookPayload } from './UserWebhookService.js';
 
 @Injectable()
 export class QueueService {
@@ -468,10 +469,10 @@ export class QueueService {
 	 * @see UserWebhookDeliverProcessorService
 	 */
 	@bindThis
-	public userWebhookDeliver(
+	public userWebhookDeliver<T extends WebhookEventTypes>(
 		webhook: MiWebhook,
-		type: typeof webhookEventTypes[number],
-		content: unknown,
+		type: T,
+		content: UserWebhookPayload<T>,
 		opts?: { attempts?: number },
 	) {
 		const data: UserWebhookDeliverJobData = {
diff --git a/packages/backend/src/core/UserWebhookService.ts b/packages/backend/src/core/UserWebhookService.ts
index 8a40a5368..7117a3d7f 100644
--- a/packages/backend/src/core/UserWebhookService.ts
+++ b/packages/backend/src/core/UserWebhookService.ts
@@ -6,11 +6,23 @@
 import { Inject, Injectable } from '@nestjs/common';
 import * as Redis from 'ioredis';
 import { type WebhooksRepository } from '@/models/_.js';
-import { MiWebhook } from '@/models/Webhook.js';
+import { MiWebhook, WebhookEventTypes } from '@/models/Webhook.js';
 import { DI } from '@/di-symbols.js';
 import { bindThis } from '@/decorators.js';
 import { GlobalEvents } from '@/core/GlobalEventService.js';
 import type { OnApplicationShutdown } from '@nestjs/common';
+import type { Packed } from '@/misc/json-schema.js';
+
+export type UserWebhookPayload<T extends WebhookEventTypes> =
+	T extends 'note' | 'reply' | 'renote' |'mention' ? {
+		note: Packed<'Note'>,
+	} :
+	T extends 'follow' | 'unfollow' ? {
+		user: Packed<'UserDetailedNotMe'>,
+	} :
+	T extends 'followed' ? {
+		user: Packed<'UserLite'>,
+	} : never;
 
 @Injectable()
 export class UserWebhookService implements OnApplicationShutdown {
diff --git a/packages/backend/src/core/WebhookTestService.ts b/packages/backend/src/core/WebhookTestService.ts
index c826a2896..b1ea7974f 100644
--- a/packages/backend/src/core/WebhookTestService.ts
+++ b/packages/backend/src/core/WebhookTestService.ts
@@ -10,7 +10,7 @@ import { MiSystemWebhook, type SystemWebhookEventType } from '@/models/SystemWeb
 import { SystemWebhookService } from '@/core/SystemWebhookService.js';
 import { Packed } from '@/misc/json-schema.js';
 import { type WebhookEventTypes } from '@/models/Webhook.js';
-import { UserWebhookService } from '@/core/UserWebhookService.js';
+import { type UserWebhookPayload, UserWebhookService } from '@/core/UserWebhookService.js';
 import { QueueService } from '@/core/QueueService.js';
 import { ModeratorInactivityRemainingTime } from '@/queue/processors/CheckModeratorsActivityProcessorService.js';
 
@@ -306,10 +306,10 @@ export class WebhookTestService {
 	 * - 送信対象イベント（on）に関する設定
 	 */
 	@bindThis
-	public async testUserWebhook(
+	public async testUserWebhook<T extends WebhookEventTypes>(
 		params: {
 			webhookId: MiWebhook['id'],
-			type: WebhookEventTypes,
+			type: T,
 			override?: Partial<Omit<MiWebhook, 'id'>>,
 		},
 		sender: MiUser | null,
@@ -321,7 +321,7 @@ export class WebhookTestService {
 		}
 
 		const webhook = webhooks[0];
-		const send = (contents: unknown) => {
+		const send = <U extends WebhookEventTypes>(type: U, contents: UserWebhookPayload<U>) => {
 			const merged = {
 				...webhook,
 				...params.override,
@@ -329,7 +329,7 @@ export class WebhookTestService {
 
 			// テスト目的なのでUserWebhookServiceの機能を経由せず直接キューに追加する（チェック処理などをスキップする意図）.
 			// また、Jobの試行回数も1回だけ.
-			this.queueService.userWebhookDeliver(merged, params.type, contents, { attempts: 1 });
+			this.queueService.userWebhookDeliver(merged, type, contents, { attempts: 1 });
 		};
 
 		const dummyNote1 = generateDummyNote({
@@ -361,33 +361,40 @@ export class WebhookTestService {
 
 		switch (params.type) {
 			case 'note': {
-				send(toPackedNote(dummyNote1));
+				send('note', { note: toPackedNote(dummyNote1) });
 				break;
 			}
 			case 'reply': {
-				send(toPackedNote(dummyReply1));
+				send('reply', { note: toPackedNote(dummyReply1) });
 				break;
 			}
 			case 'renote': {
-				send(toPackedNote(dummyRenote1));
+				send('renote', { note: toPackedNote(dummyRenote1) });
 				break;
 			}
 			case 'mention': {
-				send(toPackedNote(dummyMention1));
+				send('mention', { note: toPackedNote(dummyMention1) });
 				break;
 			}
 			case 'follow': {
-				send(toPackedUserDetailedNotMe(dummyUser1));
+				send('follow', { user: toPackedUserDetailedNotMe(dummyUser1) });
 				break;
 			}
 			case 'followed': {
-				send(toPackedUserLite(dummyUser2));
+				send('followed', { user: toPackedUserLite(dummyUser2) });
 				break;
 			}
 			case 'unfollow': {
-				send(toPackedUserDetailedNotMe(dummyUser3));
+				send('unfollow', { user: toPackedUserDetailedNotMe(dummyUser3) });
 				break;
 			}
+			// まだ実装されていない (#9485)
+			case 'reaction': return;
+			default: {
+				// eslint-disable-next-line @typescript-eslint/no-unused-vars
+				const _exhaustiveAssertion: never = params.type;
+				return;
+			}
 		}
 	}
 
diff --git a/packages/backend/test/unit/WebhookTestService.ts b/packages/backend/test/unit/WebhookTestService.ts
index 5e63b86f8..be84ae9b8 100644
--- a/packages/backend/test/unit/WebhookTestService.ts
+++ b/packages/backend/test/unit/WebhookTestService.ts
@@ -7,7 +7,7 @@
 import { Test, TestingModule } from '@nestjs/testing';
 import { beforeAll, describe, jest } from '@jest/globals';
 import { WebhookTestService } from '@/core/WebhookTestService.js';
-import { UserWebhookService } from '@/core/UserWebhookService.js';
+import { UserWebhookPayload, UserWebhookService } from '@/core/UserWebhookService.js';
 import { SystemWebhookService } from '@/core/SystemWebhookService.js';
 import { GlobalModule } from '@/GlobalModule.js';
 import { MiSystemWebhook, MiUser, MiWebhook, UserProfilesRepository, UsersRepository } from '@/models/_.js';
@@ -122,7 +122,7 @@ describe('WebhookTestService', () => {
 			const calls = queueService.userWebhookDeliver.mock.calls[0];
 			expect((calls[0] as any).id).toBe('dummy-webhook');
 			expect(calls[1]).toBe('note');
-			expect((calls[2] as any).id).toBe('dummy-note-1');
+			expect((calls[2] as UserWebhookPayload<'note'>).note.id).toBe('dummy-note-1');
 		});
 
 		test('reply', async () => {
@@ -131,7 +131,7 @@ describe('WebhookTestService', () => {
 			const calls = queueService.userWebhookDeliver.mock.calls[0];
 			expect((calls[0] as any).id).toBe('dummy-webhook');
 			expect(calls[1]).toBe('reply');
-			expect((calls[2] as any).id).toBe('dummy-reply-1');
+			expect((calls[2] as UserWebhookPayload<'reply'>).note.id).toBe('dummy-reply-1');
 		});
 
 		test('renote', async () => {
@@ -140,7 +140,7 @@ describe('WebhookTestService', () => {
 			const calls = queueService.userWebhookDeliver.mock.calls[0];
 			expect((calls[0] as any).id).toBe('dummy-webhook');
 			expect(calls[1]).toBe('renote');
-			expect((calls[2] as any).id).toBe('dummy-renote-1');
+			expect((calls[2] as UserWebhookPayload<'renote'>).note.id).toBe('dummy-renote-1');
 		});
 
 		test('mention', async () => {
@@ -149,7 +149,7 @@ describe('WebhookTestService', () => {
 			const calls = queueService.userWebhookDeliver.mock.calls[0];
 			expect((calls[0] as any).id).toBe('dummy-webhook');
 			expect(calls[1]).toBe('mention');
-			expect((calls[2] as any).id).toBe('dummy-mention-1');
+			expect((calls[2] as UserWebhookPayload<'mention'>).note.id).toBe('dummy-mention-1');
 		});
 
 		test('follow', async () => {
@@ -158,7 +158,7 @@ describe('WebhookTestService', () => {
 			const calls = queueService.userWebhookDeliver.mock.calls[0];
 			expect((calls[0] as any).id).toBe('dummy-webhook');
 			expect(calls[1]).toBe('follow');
-			expect((calls[2] as any).id).toBe('dummy-user-1');
+			expect((calls[2] as UserWebhookPayload<'follow'>).user.id).toBe('dummy-user-1');
 		});
 
 		test('followed', async () => {
@@ -167,7 +167,7 @@ describe('WebhookTestService', () => {
 			const calls = queueService.userWebhookDeliver.mock.calls[0];
 			expect((calls[0] as any).id).toBe('dummy-webhook');
 			expect(calls[1]).toBe('followed');
-			expect((calls[2] as any).id).toBe('dummy-user-2');
+			expect((calls[2] as UserWebhookPayload<'followed'>).user.id).toBe('dummy-user-2');
 		});
 
 		test('unfollow', async () => {
@@ -176,7 +176,7 @@ describe('WebhookTestService', () => {
 			const calls = queueService.userWebhookDeliver.mock.calls[0];
 			expect((calls[0] as any).id).toBe('dummy-webhook');
 			expect(calls[1]).toBe('unfollow');
-			expect((calls[2] as any).id).toBe('dummy-user-3');
+			expect((calls[2] as UserWebhookPayload<'unfollow'>).user.id).toBe('dummy-user-3');
 		});
 
 		describe('NoSuchWebhookError', () => {
diff --git a/packages/frontend/src/pages/settings/webhook.edit.vue b/packages/frontend/src/pages/settings/webhook.edit.vue
index 40d23e36c..22b008fb6 100644
--- a/packages/frontend/src/pages/settings/webhook.edit.vue
+++ b/packages/frontend/src/pages/settings/webhook.edit.vue
@@ -44,7 +44,7 @@ SPDX-License-Identifier: AGPL-3.0-only
 					<MkButton transparent :class="$style.testButton" :disabled="!(active && event_renote)" @click="test('renote')"><i class="ti ti-send"></i></MkButton>
 				</div>
 				<div :class="$style.switchBox">
-					<MkSwitch v-model="event_reaction">{{ i18n.ts._webhookSettings._events.reaction }}</MkSwitch>
+					<MkSwitch v-model="event_reaction" :disabled="true">{{ i18n.ts._webhookSettings._events.reaction }}</MkSwitch>
 					<MkButton transparent :class="$style.testButton" :disabled="!(active && event_reaction)" @click="test('reaction')"><i class="ti ti-send"></i></MkButton>
 				</div>
 				<div :class="$style.switchBox">
diff --git a/packages/frontend/src/pages/settings/webhook.new.vue b/packages/frontend/src/pages/settings/webhook.new.vue
index d62357caa..727c4df2d 100644
--- a/packages/frontend/src/pages/settings/webhook.new.vue
+++ b/packages/frontend/src/pages/settings/webhook.new.vue
@@ -27,7 +27,7 @@ SPDX-License-Identifier: AGPL-3.0-only
 			<MkSwitch v-model="event_note">{{ i18n.ts._webhookSettings._events.note }}</MkSwitch>
 			<MkSwitch v-model="event_reply">{{ i18n.ts._webhookSettings._events.reply }}</MkSwitch>
 			<MkSwitch v-model="event_renote">{{ i18n.ts._webhookSettings._events.renote }}</MkSwitch>
-			<MkSwitch v-model="event_reaction">{{ i18n.ts._webhookSettings._events.reaction }}</MkSwitch>
+			<MkSwitch v-model="event_reaction" :disabled="true">{{ i18n.ts._webhookSettings._events.reaction }}</MkSwitch>
 			<MkSwitch v-model="event_mention">{{ i18n.ts._webhookSettings._events.mention }}</MkSwitch>
 		</div>
 	</FormSection>