diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3853b76e4dc..38e4e9ce7c8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,7 +12,7 @@
 ### Server
 - Enhance: リモートノートクリーニングジョブのスキップ処理のパフォーマンス改善
 - Fix: backend バンドルで `@tensorflow/tfjs-node` を external に含めず、起動時に `@mapbox/node-pre-gyp` の `find()` が backend の package.json を誤検出して `is not node-pre-gyp ready` エラーを永続的に吐く問題を修正
-
+- Fix: MemoryKVCacheのキャッシュGC処理において、更新されたキャッシュが期限切れにならないことがある問題を修正
 
 ## 2026.5.4
 
diff --git a/packages/backend/src/misc/cache.ts b/packages/backend/src/misc/cache.ts
index f9692ce5d5f..c2dc1033d09 100644
--- a/packages/backend/src/misc/cache.ts
+++ b/packages/backend/src/misc/cache.ts
@@ -220,6 +220,10 @@ export class MemoryKVCache<T> {
 	 * @deprecated これを直接呼び出すべきではない。InternalEventなどで変更を全てのプロセス/マシンに通知するべき
 	 */
 	public set(key: string, value: T): void {
+		// Delete before re-inserting so the Map preserves insertion order for gc().
+		// Without this, an updated key stays at its original position, which breaks
+		// the "oldest-first" assumption that gc() relies on to stop early.
+		this.cache.delete(key);
 		this.cache.set(key, {
 			date: Date.now(),
 			value,
diff --git a/packages/backend/test/unit/misc/cache.ts b/packages/backend/test/unit/misc/cache.ts
new file mode 100644
index 00000000000..9ba7ee10f7d
--- /dev/null
+++ b/packages/backend/test/unit/misc/cache.ts
@@ -0,0 +1,216 @@
+/*
+ * SPDX-FileCopyrightText: syuilo and misskey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
+import { MemoryKVCache, MemorySingleCache } from '@/misc/cache.js';
+
+describe('misc:MemoryKVCache', () => {
+	beforeEach(() => {
+		vi.useFakeTimers();
+	});
+
+	afterEach(() => {
+		vi.useRealTimers();
+	});
+
+	test('set and get returns the value within lifetime', () => {
+		const cache = new MemoryKVCache<string>(1000);
+		cache.set('key', 'value');
+		expect(cache.get('key')).toBe('value');
+		cache.dispose();
+	});
+
+	test('get returns undefined after lifetime expires', () => {
+		const cache = new MemoryKVCache<string>(1000);
+		cache.set('key', 'value');
+		vi.advanceTimersByTime(1001);
+		expect(cache.get('key')).toBeUndefined();
+		cache.dispose();
+	});
+
+	test('delete removes the entry', () => {
+		const cache = new MemoryKVCache<string>(1000);
+		cache.set('key', 'value');
+		cache.delete('key');
+		expect(cache.get('key')).toBeUndefined();
+		cache.dispose();
+	});
+
+	describe('gc()', () => {
+		test('removes expired entries', () => {
+			const cache = new MemoryKVCache<string>(1000);
+			cache.set('a', '1');
+			cache.set('b', '2');
+			vi.advanceTimersByTime(1001);
+			cache.gc();
+			expect(cache.get('a')).toBeUndefined();
+			expect(cache.get('b')).toBeUndefined();
+			cache.dispose();
+		});
+
+		test('retains entries that have not yet expired', () => {
+			const cache = new MemoryKVCache<string>(2000);
+			cache.set('a', '1');
+			vi.advanceTimersByTime(1001);
+			cache.gc();
+			expect(cache.get('a')).toBe('1');
+			cache.dispose();
+		});
+
+		test('removes only expired entries when mixed with live entries', () => {
+			const cache = new MemoryKVCache<string>(2000);
+			cache.set('old', 'oldValue');
+			vi.advanceTimersByTime(2001);
+			cache.set('new', 'newValue');
+			cache.gc();
+			expect(cache.get('old')).toBeUndefined();
+			expect(cache.get('new')).toBe('newValue');
+			cache.dispose();
+		});
+
+		// Regression test for https://github.com/misskey-dev/misskey/issues/15500
+		// set() previously did NOT delete-before-reinsert, so updated keys kept their
+		// original insertion position. gc() would then stop early at the updated key
+		// (whose timestamp looked fresh) and leave subsequent, truly-expired keys alive.
+		// The key observable symptom is that gc() fails to *remove* the expired entry
+		// from the Map — get() has its own expiry check so it returns undefined either
+		// way, but the stale entry keeps consuming memory.
+		test('correctly expires old entries after a key is updated (issue #15500)', () => {
+			const lifetime = 1000;
+			const cache = new MemoryKVCache<string>(lifetime);
+
+			// Insert 'a' and 'b' at t=0
+			cache.set('a', 'v1');
+			cache.set('b', 'v1');
+
+			// Advance time and update 'a' — without the fix, 'a' stays at position 0
+			// in the Map, so gc() sees it as fresh first and breaks early, leaving 'b'
+			// (which is also expired) still in the Map (though get() would hide it).
+			vi.advanceTimersByTime(500);
+			cache.set('a', 'v2'); // refresh 'a'; 'b' is still at t=0
+
+			// 'b' is now expired, 'a' has 400ms left
+			vi.advanceTimersByTime(600); // total 1100ms
+
+			cache.gc();
+
+			// Verify the entry is actually removed from the Map, not just hidden by get().
+			// get() always checks expiry itself, so it returns undefined even without gc() —
+			// the real bug is memory not being freed.
+			const entries = [...cache.entries];
+			expect(entries.find(([k]) => k === 'b')).toBeUndefined(); // 'b' must be gone from Map
+			expect(entries.find(([k]) => k === 'a')?.[1].value).toBe('v2'); // 'a' still in Map
+			cache.dispose();
+		});
+
+		test('gc does not break when cache is empty', () => {
+			const cache = new MemoryKVCache<string>(1000);
+			expect(() => cache.gc()).not.toThrow();
+			cache.dispose();
+		});
+	});
+
+	describe('fetch()', () => {
+		test('calls fetcher on cache miss', async () => {
+			const cache = new MemoryKVCache<string>(1000);
+			const fetcher = vi.fn().mockResolvedValue('fetched');
+			const result = await cache.fetch('key', fetcher);
+			expect(fetcher).toHaveBeenCalledOnce();
+			expect(result).toBe('fetched');
+			cache.dispose();
+		});
+
+		test('does not call fetcher on cache hit', async () => {
+			const cache = new MemoryKVCache<string>(1000);
+			cache.set('key', 'cached');
+			const fetcher = vi.fn().mockResolvedValue('fetched');
+			const result = await cache.fetch('key', fetcher);
+			expect(fetcher).not.toHaveBeenCalled();
+			expect(result).toBe('cached');
+			cache.dispose();
+		});
+
+		test('respects validator and bypasses cache when validator returns false', async () => {
+			const cache = new MemoryKVCache<string>(1000);
+			cache.set('key', 'cached');
+			const fetcher = vi.fn().mockResolvedValue('fetched');
+			const result = await cache.fetch('key', fetcher, () => false);
+			expect(fetcher).toHaveBeenCalledOnce();
+			expect(result).toBe('fetched');
+			cache.dispose();
+		});
+	});
+
+	describe('fetchMaybe()', () => {
+		test('does not cache undefined returned by fetcher', async () => {
+			const cache = new MemoryKVCache<string>(1000);
+			const fetcher = vi.fn().mockResolvedValue(undefined);
+			const result = await cache.fetchMaybe('key', fetcher);
+			expect(result).toBeUndefined();
+			// A second call should invoke the fetcher again because undefined was not cached
+			await cache.fetchMaybe('key', fetcher);
+			expect(fetcher).toHaveBeenCalledTimes(2);
+			cache.dispose();
+		});
+	});
+});
+
+describe('misc:MemorySingleCache', () => {
+	beforeEach(() => {
+		vi.useFakeTimers();
+	});
+
+	afterEach(() => {
+		vi.useRealTimers();
+	});
+
+	test('set and get returns the value within lifetime', () => {
+		const cache = new MemorySingleCache<string>(1000);
+		cache.set('value');
+		expect(cache.get()).toBe('value');
+	});
+
+	test('get returns undefined after lifetime expires', () => {
+		const cache = new MemorySingleCache<string>(1000);
+		cache.set('value');
+		vi.advanceTimersByTime(1001);
+		expect(cache.get()).toBeUndefined();
+	});
+
+	test('delete removes the cached value', () => {
+		const cache = new MemorySingleCache<string>(1000);
+		cache.set('value');
+		cache.delete();
+		expect(cache.get()).toBeUndefined();
+	});
+
+	describe('fetch()', () => {
+		test('calls fetcher on cache miss', async () => {
+			const cache = new MemorySingleCache<string>(1000);
+			const fetcher = vi.fn().mockResolvedValue('fetched');
+			const result = await cache.fetch(fetcher);
+			expect(fetcher).toHaveBeenCalledOnce();
+			expect(result).toBe('fetched');
+		});
+
+		test('does not call fetcher on cache hit', async () => {
+			const cache = new MemorySingleCache<string>(1000);
+			cache.set('cached');
+			const fetcher = vi.fn().mockResolvedValue('fetched');
+			const result = await cache.fetch(fetcher);
+			expect(fetcher).not.toHaveBeenCalled();
+			expect(result).toBe('cached');
+		});
+
+		test('respects validator and bypasses cache when validator returns false', async () => {
+			const cache = new MemorySingleCache<string>(1000);
+			cache.set('cached');
+			const fetcher = vi.fn().mockResolvedValue('fetched');
+			const result = await cache.fetch(fetcher, () => false);
+			expect(fetcher).toHaveBeenCalledOnce();
+			expect(result).toBe('fetched');
+		});
+	});
+});