diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3341d4afff..8ce566370b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@
 - チャート生成時にinstance.suspentionStateに置き換えられたinstance.isSuspendedが参照されてしまう問題を修正
 - Feat: レートリミット制限に引っかかったときに`Retry-After`ヘッダーを返すように (#13949)
 - Fix: アンテナ・クリップ・リスト・ウェブフックがロールポリシーの上限より一つ多く作れてしまうのを修正 (#14036)
+- Fix: FTT有効時、タイムライン用エンドポイントで`sinceId`にキャッシュ内最古のものより古いものを指定した場合に正しく結果が返ってこない問題を修正
 
 ## 2024.5.0
 
diff --git a/packages/backend/src/core/FanoutTimelineEndpointService.ts b/packages/backend/src/core/FanoutTimelineEndpointService.ts
index d5058f37c2..b05af99c5e 100644
--- a/packages/backend/src/core/FanoutTimelineEndpointService.ts
+++ b/packages/backend/src/core/FanoutTimelineEndpointService.ts
@@ -55,9 +55,6 @@ export class FanoutTimelineEndpointService {
 
 	@bindThis
 	private async getMiNotes(ps: TimelineOptions): Promise<MiNote[]> {
-		let noteIds: string[];
-		let shouldFallbackToDb = false;
-
 		// 呼び出し元と以下の処理をシンプルにするためにdbFallbackを置き換える
 		if (!ps.useDbFallback) ps.dbFallback = () => Promise.resolve([]);
 
@@ -67,12 +64,11 @@ export class FanoutTimelineEndpointService {
 		const redisResult = await this.fanoutTimelineService.getMulti(ps.redisTimelines, ps.untilId, ps.sinceId);
 
 		// TODO: いい感じにgetMulti内でソート済だからuniqするときにredisResultが全てソート済なのを利用して再ソートを避けたい
-		const redisResultIds = Array.from(new Set(redisResult.flat(1)));
+		const redisResultIds = Array.from(new Set(redisResult.flat(1))).sort(idCompare);
 
-		redisResultIds.sort(idCompare);
-		noteIds = redisResultIds.slice(0, ps.limit);
-
-		shouldFallbackToDb = shouldFallbackToDb || (noteIds.length === 0);
+		let noteIds = redisResultIds.slice(0, ps.limit);
+		const oldestNoteId = ascending ? redisResultIds[0] : redisResultIds[redisResultIds.length - 1];
+		const shouldFallbackToDb = noteIds.length === 0 || ps.sinceId != null && ps.sinceId < oldestNoteId;
 
 		if (!shouldFallbackToDb) {
 			let filter = ps.noteFilter ?? (_note => true);
diff --git a/packages/backend/test/e2e/timelines.ts b/packages/backend/test/e2e/timelines.ts
index 5487292afc..f6cc2bac28 100644
--- a/packages/backend/test/e2e/timelines.ts
+++ b/packages/backend/test/e2e/timelines.ts
@@ -7,6 +7,8 @@
 // pnpm jest -- e2e/timelines.ts
 
 import * as assert from 'assert';
+import { Redis } from 'ioredis';
+import { loadConfig } from '@/config.js';
 import { api, post, randomString, sendEnvUpdateRequest, signup, sleep, uploadUrl } from '../utils.js';
 
 function genHost() {
@@ -17,7 +19,13 @@ function waitForPushToTl() {
 	return sleep(500);
 }
 
+let redisForTimelines: Redis;
+
 describe('Timelines', () => {
+	beforeAll(() => {
+		redisForTimelines = new Redis(loadConfig().redisForTimelines);
+	});
+
 	describe('Home TL', () => {
 		test.concurrent('自分の visibility: followers なノートが含まれる', async () => {
 			const [alice] = await Promise.all([signup()]);
@@ -1272,6 +1280,33 @@ describe('Timelines', () => {
 
 			assert.strictEqual(res.body.some((note: any) => note.id === bobNote.id), false);
 		});
+
+		/** @see https://github.com/misskey-dev/misskey/issues/14000 */
+		test.concurrent('FTT: sinceId にキャッシュより古いノートを指定しても、sinceId による絞り込みが正しく動作する', async () => {
+			const alice = await signup();
+			const noteSince = await post(alice, { text: 'Note where id will be `sinceId`.' });
+			const note1 = await post(alice, { text: '1' });
+			const note2 = await post(alice, { text: '2' });
+			await redisForTimelines.del('list:userTimeline:' + alice.id);
+			const note3 = await post(alice, { text: '3' });
+
+			const res = await api('users/notes', { userId: alice.id, sinceId: noteSince.id });
+			assert.deepStrictEqual(res.body, [note1, note2, note3]);
+		});
+
+		test.concurrent('FTT: sinceId にキャッシュより古いノートを指定しても、sinceId と untilId による絞り込みが正しく動作する', async () => {
+			const alice = await signup();
+			const noteSince = await post(alice, { text: 'Note where id will be `sinceId`.' });
+			const note1 = await post(alice, { text: '1' });
+			const note2 = await post(alice, { text: '2' });
+			await redisForTimelines.del('list:userTimeline:' + alice.id);
+			const note3 = await post(alice, { text: '3' });
+			const noteUntil = await post(alice, { text: 'Note where id will be `untilId`.' });
+			await post(alice, { text: '4' });
+
+			const res = await api('users/notes', { userId: alice.id, sinceId: noteSince.id, untilId: noteUntil.id });
+			assert.deepStrictEqual(res.body, [note3, note2, note1]);
+		});
 	});
 
 	// TODO: リノートミュート済みユーザーのテスト