diff --git a/.changeset/perky-sloths-say.md b/.changeset/perky-sloths-say.md
new file mode 100644
index 000000000..ce29f2daa
--- /dev/null
+++ b/.changeset/perky-sloths-say.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/core': patch
+---
+
+Allow overriding the default request timeout via `MCP_REQUEST_TIMEOUT_MSEC` environment variable
diff --git a/docs/client.md b/docs/client.md
index b5086f531..8163370f8 100644
--- a/docs/client.md
+++ b/docs/client.md
@@ -539,7 +539,7 @@ client.onclose = () => {
 
 ### Timeouts
 
-All requests have a 60-second default timeout. Pass a custom `timeout` in the options to override it. On timeout, the SDK sends a cancellation notification to the server and rejects the promise with {@linkcode @modelcontextprotocol/client!index.SdkErrorCode.RequestTimeout | SdkErrorCode.RequestTimeout}:
+All requests have a 60-second default timeout. You can override it globally by setting the `MCP_REQUEST_TIMEOUT_MSEC` environment variable (read once at module load; must be a positive integer up to 43,200,000 / 12 hours), or pass a custom `timeout` in the options per request. On timeout, the SDK sends a cancellation notification to the server and rejects the promise with {@linkcode @modelcontextprotocol/client!index.SdkErrorCode.RequestTimeout | SdkErrorCode.RequestTimeout}:
 
 ```ts source="../examples/client/src/clientGuide.examples.ts#errorHandling_timeout"
 try {
diff --git a/packages/core/src/shared/protocol.ts b/packages/core/src/shared/protocol.ts
index 57eab6932..e4ba0148a 100644
--- a/packages/core/src/shared/protocol.ts
+++ b/packages/core/src/shared/protocol.ts
@@ -94,10 +94,28 @@ export type ProtocolOptions = {
     tasks?: TaskManagerOptions;
 };
 
+const MAX_REQUEST_TIMEOUT_MSEC = 43_200_000; // 12 hours
+const DEFAULT_TIMEOUT_FALLBACK = 60_000;
+
+/**
+ * Resolves the request timeout from the environment variable `MCP_REQUEST_TIMEOUT_MSEC`.
+ * Exported for testing; not part of the public API.
+ * @internal
+ */
+export function resolveRequestTimeout(): number {
+    const raw = typeof process !== 'undefined' && process?.env ? process.env.MCP_REQUEST_TIMEOUT_MSEC : undefined;
+    const parsed = Number.parseInt(raw ?? '', 10);
+    return parsed > 0 && parsed <= MAX_REQUEST_TIMEOUT_MSEC ? parsed : DEFAULT_TIMEOUT_FALLBACK;
+}
+
 /**
  * The default request timeout, in milliseconds.
+ *
+ * Can be overridden via the `MCP_REQUEST_TIMEOUT_MSEC` environment variable.
+ * The value is read once at module load time; changes after import have no effect.
+ * Must be a positive integer no greater than 43,200,000 (12 hours).
  */
-export const DEFAULT_REQUEST_TIMEOUT_MSEC = 60_000;
+export const DEFAULT_REQUEST_TIMEOUT_MSEC = resolveRequestTimeout();
 
 /**
  * Options that can be given per request.
diff --git a/packages/core/test/shared/defaultRequestTimeout.test.ts b/packages/core/test/shared/defaultRequestTimeout.test.ts
new file mode 100644
index 000000000..a1a919ee2
--- /dev/null
+++ b/packages/core/test/shared/defaultRequestTimeout.test.ts
@@ -0,0 +1,108 @@
+import { vi, describe, test, expect, afterEach } from 'vitest';
+import { resolveRequestTimeout } from '../../src/shared/protocol.js';
+
+/**
+ * DEFAULT_REQUEST_TIMEOUT_MSEC is computed once at module load via an IIFE,
+ * so each scenario needs a fresh import. We use `vi.resetModules()` +
+ * dynamic `import()` to re-evaluate the module with different env state.
+ *
+ * For tests that stub `process` itself (undefined/null), we call
+ * `resolveRequestTimeout()` directly — a full dynamic import would fail
+ * because transitive dependencies (e.g. zod) also read `process`.
+ */
+
+afterEach(() => {
+    vi.unstubAllGlobals();
+    vi.unstubAllEnvs();
+    vi.resetModules();
+});
+
+async function loadDefault(): Promise<number> {
+    const mod = await import('../../src/shared/protocol.js');
+    return mod.DEFAULT_REQUEST_TIMEOUT_MSEC;
+}
+
+describe('DEFAULT_REQUEST_TIMEOUT_MSEC', () => {
+    test('falls back to 60_000 when env var is not set', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', '');
+        expect(await loadDefault()).toBe(60_000);
+    });
+
+    test('uses valid numeric env var', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', '120000');
+        expect(await loadDefault()).toBe(120_000);
+    });
+
+    test('falls back to 60_000 for empty string', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', '');
+        expect(await loadDefault()).toBe(60_000);
+    });
+
+    test('falls back to 60_000 for non-numeric string', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', 'abc');
+        expect(await loadDefault()).toBe(60_000);
+    });
+
+    test('falls back to 60_000 for negative number', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', '-5000');
+        expect(await loadDefault()).toBe(60_000);
+    });
+
+    test('falls back to 60_000 for zero', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', '0');
+        expect(await loadDefault()).toBe(60_000);
+    });
+
+    test('falls back to 60_000 for undefined', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', undefined);
+        expect(await loadDefault()).toBe(60_000);
+    });
+
+    test('falls back to 60_000 for null', async () => {
+        // @ts-expect-error -- testing runtime behavior with null
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', null);
+        expect(await loadDefault()).toBe(60_000);
+    });
+
+    test('falls back to 60_000 for value exceeding Number.MAX_SAFE_INTEGER', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', '9007199254740993');
+        expect(await loadDefault()).toBe(60_000);
+    });
+
+    test('caps at 12-hour upper bound', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', '43200001');
+        expect(await loadDefault()).toBe(60_000);
+    });
+
+    test('accepts exactly 12 hours (43200000)', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', '43200000');
+        expect(await loadDefault()).toBe(43_200_000);
+    });
+
+    test('falls back to 60_000 for extremely large value', async () => {
+        vi.stubEnv('MCP_REQUEST_TIMEOUT_MSEC', '999999999');
+        expect(await loadDefault()).toBe(60_000);
+    });
+
+    test('falls back to 60_000 when process is undefined', () => {
+        vi.stubGlobal('process', undefined);
+        expect(resolveRequestTimeout()).toBe(60_000);
+    });
+
+    test('falls back to 60_000 when process is null', () => {
+        vi.stubGlobal('process', null);
+        expect(resolveRequestTimeout()).toBe(60_000);
+    });
+
+    test('falls back to 60_000 when process.env is undefined', () => {
+        const original = globalThis.process;
+        vi.stubGlobal('process', { ...original, env: undefined });
+        expect(resolveRequestTimeout()).toBe(60_000);
+    });
+
+    test('falls back to 60_000 when process.env is null', () => {
+        const original = globalThis.process;
+        vi.stubGlobal('process', { ...original, env: null });
+        expect(resolveRequestTimeout()).toBe(60_000);
+    });
+});