diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4bcf30b603..aaf2234cc5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -71,6 +71,7 @@
 - Fix: フォロワー限定投稿へのリプライをホーム投稿に出来る問題を修正
 - Fix: ファイルをアップロードするAPIにて、処理終了後に一時ファイルが削除されないことがある問題を修正
 - Fix: 初期設定で作成したアカウント以外でアカウント作成APIが使用できない問題を修正
+- Fix: フォロー中のチャンネルを再度フォローした際にALREADY_FOLLOWINGエラーを返すように
 
 ## 2026.6.0
 
diff --git a/packages/backend/src/core/ChannelFollowingService.ts b/packages/backend/src/core/ChannelFollowingService.ts
index 7dae8d10f3..54198fdc06 100644
--- a/packages/backend/src/core/ChannelFollowingService.ts
+++ b/packages/backend/src/core/ChannelFollowingService.ts
@@ -13,6 +13,8 @@ import { GlobalEvents, GlobalEventService } from '@/core/GlobalEventService.js';
 import { bindThis } from '@/decorators.js';
 import type { MiLocalUser } from '@/models/User.js';
 import { RedisKVCache } from '@/misc/cache.js';
+import { IdentifiableError } from '@/misc/identifiable-error.js';
+import { isDuplicateKeyValueError } from '@/misc/is-duplicate-key-value-error.js';
 
 @Injectable()
 export class ChannelFollowingService implements OnModuleInit {
@@ -96,11 +98,18 @@ export class ChannelFollowingService implements OnModuleInit {
 		requestUser: MiLocalUser,
 		targetChannel: MiChannel,
 	): Promise<void> {
-		await this.channelFollowingsRepository.insert({
-			id: this.idService.gen(),
-			followerId: requestUser.id,
-			followeeId: targetChannel.id,
-		});
+		try {
+			await this.channelFollowingsRepository.insert({
+				id: this.idService.gen(),
+				followerId: requestUser.id,
+				followeeId: targetChannel.id,
+			});
+		} catch (e) {
+			if (isDuplicateKeyValueError(e)) {
+				throw new IdentifiableError('6e335e39-0203-4418-a936-b3f2dc987845', 'already following');
+			}
+			throw e;
+		}
 
 		this.globalEventService.publishInternalEvent('followChannel', {
 			userId: requestUser.id,
diff --git a/packages/backend/src/server/api/endpoints/channels/follow.ts b/packages/backend/src/server/api/endpoints/channels/follow.ts
index 1812820ba2..b46552281f 100644
--- a/packages/backend/src/server/api/endpoints/channels/follow.ts
+++ b/packages/backend/src/server/api/endpoints/channels/follow.ts
@@ -8,6 +8,7 @@ import { Endpoint } from '@/server/api/endpoint-base.js';
 import type { ChannelsRepository } from '@/models/_.js';
 import { DI } from '@/di-symbols.js';
 import { ChannelFollowingService } from '@/core/ChannelFollowingService.js';
+import { IdentifiableError } from '@/misc/identifiable-error.js';
 import { ApiError } from '../../error.js';
 
 export const meta = {
@@ -25,6 +26,11 @@ export const meta = {
 			code: 'NO_SUCH_CHANNEL',
 			id: 'c0031718-d573-4e85-928e-10039f1fbb68',
 		},
+		alreadyFollowing: {
+			message: 'You are already following that channel.',
+			code: 'ALREADY_FOLLOWING',
+			id: '7db31665-651e-40c1-8e6e-28e9ad829a2d',
+		},
 	},
 } as const;
 
@@ -52,7 +58,14 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				throw new ApiError(meta.errors.noSuchChannel);
 			}
 
-			await this.channelFollowingService.follow(me, channel);
+			try {
+				await this.channelFollowingService.follow(me, channel);
+			} catch (e) {
+				if (e instanceof IdentifiableError) {
+					if (e.id === '6e335e39-0203-4418-a936-b3f2dc987845') throw new ApiError(meta.errors.alreadyFollowing);
+				}
+				throw e;
+			}
 		});
 	}
 }
diff --git a/packages/backend/test/e2e/channel.ts b/packages/backend/test/e2e/channel.ts
new file mode 100644
index 0000000000..c2fd98cbf4
--- /dev/null
+++ b/packages/backend/test/e2e/channel.ts
@@ -0,0 +1,36 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+process.env.NODE_ENV = 'test';
+
+import * as assert from 'assert';
+import { describe, beforeAll, test } from 'vitest';
+import { api, castAsError, signup } from '../utils.js';
+import type * as misskey from 'misskey-js';
+
+describe('Channel', () => {
+	let alice: misskey.entities.SignupResponse;
+	beforeAll(async () => {
+		alice = await signup({ username: 'alice' });
+	});
+
+	describe('Follow', () => {
+		let channel: misskey.entities.ChannelsCreateResponse;
+
+		beforeAll(async () => {
+			const res = await api('channels/create', { name: 'follow-test-channel' }, alice);
+			channel = res.body;
+		});
+
+		test('フォローしているチャンネルを再度フォローするとALREADY_FOLLOWINGエラーになる', async () => {
+			const res1 = await api('channels/follow', { channelId: channel.id }, alice);
+			assert.strictEqual(res1.status, 204);
+
+			const res2 = await api('channels/follow', { channelId: channel.id }, alice);
+			assert.strictEqual(res2.status, 400);
+			assert.strictEqual(castAsError(res2.body as any).error.code, 'ALREADY_FOLLOWING');
+		});
+	});
+});