diff --git a/packages/frontend/src/preferences/manager.ts b/packages/frontend/src/preferences/manager.ts
index 12b84706bb..4b1ff1c3f3 100644
--- a/packages/frontend/src/preferences/manager.ts
+++ b/packages/frontend/src/preferences/manager.ts
@@ -75,7 +75,7 @@ export type PreferencesProfile = {
 	id: string;
 	version: string;
 	type: 'main';
-	modifiedAt: number;
+	modifiedAt: number; // 仕様が若干直感的ではない(syncされた値が降ってきたときは更新されないなど)ため、一応残してはいるが積極的な利用はしない方が無難
 	name: string;
 	preferences: {
 		[K in keyof PREF]: PrefRecord<K>[];
@@ -181,6 +181,39 @@ function normalizePreferences(preferences: PossiblyNonNormalizedPreferencesProfi
 	return data as PreferencesProfile['preferences'];
 }
 
+// 各recordについて、modifiedAtが大きい方を採用する
+export function mergeProfiles(a: PreferencesProfile, b: PreferencesProfile): PreferencesProfile {
+	const merged = {
+		...a,
+		modifiedAt: Math.max(a.modifiedAt, b.modifiedAt),
+		preferences: {},
+	} as PreferencesProfile;
+
+	for (const _key in PREF_DEF) {
+		const key = _key as keyof PREF;
+		const aRecords = a.preferences[key];
+		const bRecords = b.preferences[key];
+
+		const mergedRecords = [...aRecords];
+
+		for (const bRecord of bRecords) {
+			const existingIndex = mergedRecords.findIndex(([scope]) => isSameScope(scope, bRecord[0]));
+			if (existingIndex === -1) {
+				mergedRecords.push(bRecord);
+			} else {
+				const aRecord = mergedRecords[existingIndex];
+				if ((bRecord[2].modifiedAt ?? 0) > (aRecord[2].modifiedAt ?? 0)) {
+					mergedRecords[existingIndex] = bRecord;
+				}
+			}
+		}
+
+		(merged.preferences[key] as PrefRecord<typeof key>[]) = mergedRecords;
+	}
+
+	return merged;
+}
+
 // eslint-disable-next-line @typescript-eslint/no-empty-object-type
 type PreferencesManagerEvents = {
 };
diff --git a/packages/frontend/src/preferences/utility.ts b/packages/frontend/src/preferences/utility.ts
index c62e0cf13d..d0791271f0 100644
--- a/packages/frontend/src/preferences/utility.ts
+++ b/packages/frontend/src/preferences/utility.ts
@@ -4,6 +4,7 @@
  */
 
 import { ref, watch } from 'vue';
+import { mergeProfiles } from './manager.js';
 import type { PreferencesProfile } from './manager.js';
 import type { MenuItem } from '@/types/menu.js';
 import { copyToClipboard } from '@/utility/copy-to-clipboard.js';
@@ -180,14 +181,24 @@ export async function cloudBackup() {
 		throw new Error('cannot auto backup for this profile');
 	}
 
-	// TODO: 同期有効時、既に新しいバージョンがバックアップされている場合は上書きしないようにする
+	let currentProfile = prefer.profile;
 
-	if (_DEV_) console.log('cloud backup', prefer.profile);
+	if (_DEV_) console.log('cloud backup', currentProfile);
+
+	const backupedProfile = await misskeyApi('i/registry/get', {
+		scope: ['client', 'preferences', 'backups'],
+		key: prefer.profile.name,
+	}) as PreferencesProfile | null;
+
+	// 古い設定で新しいバックアップを上書きしないようにマージ
+	if (backupedProfile != null) {
+		currentProfile = mergeProfiles(currentProfile, backupedProfile);
+	}
 
 	await misskeyApi('i/registry/set', {
 		scope: ['client', 'preferences', 'backups'],
 		key: prefer.profile.name,
-		value: prefer.profile,
+		value: currentProfile,
 	});
 }