diff --git a/src/process/ExecFileOutput.test.ts b/src/process/ExecFileOutput.test.ts
index 00b925007..f4d690e33 100644
--- a/src/process/ExecFileOutput.test.ts
+++ b/src/process/ExecFileOutput.test.ts
@@ -123,6 +123,22 @@ describe.skipIf(process.platform === "win32")(
       expect(await readFile(stopped, "utf8")).toBe("SIGINT");
     });
 
+    it.each([-1, Number.NaN, 1.5, Number.POSITIVE_INFINITY, 2 ** 53])(
+      "refuses a malformed timeout (%s) in both modes before starting the command",
+      async (timeout) => {
+        // Starting this command would fail with ENOENT instead.
+        await expect(
+          execFileOutput("/nonexistent/rea-command", [], {
+            timeout,
+            stopSignal: "SIGINT",
+          }),
+        ).rejects.toMatchObject({ code: "ERR_OUT_OF_RANGE" });
+        await expect(
+          execFileOutput("/nonexistent/rea-command", [], { timeout }),
+        ).rejects.toMatchObject({ code: "ERR_OUT_OF_RANGE" });
+      },
+    );
+
     it("kills a timed-out process that ignores the stop signal", async () => {
       const script = [
         'for (const name of ["SIGINT", "SIGTERM"]) process.on(name, () => {});',
diff --git a/src/process/ExecFileOutput.ts b/src/process/ExecFileOutput.ts
index 294014c19..dd6aaae62 100644
--- a/src/process/ExecFileOutput.ts
+++ b/src/process/ExecFileOutput.ts
@@ -53,6 +53,21 @@ const timeoutError = (
   return error;
 };
 
+/**
+ * The `timeout` contract for both modes, checked before spawning. A stoppable
+ * command keeps its own deadline, so execFile never validates it. Node 26's
+ * execFile also caps it at Number.MAX_SAFE_INTEGER; Node 22 and 24 accept
+ * larger integers, which their timers then clamp to 1 ms.
+ */
+const invalidTimeout = (timeout: number): RangeError | undefined => {
+  if (Number.isSafeInteger(timeout) && timeout >= 0) return undefined;
+  const error = new RangeError(
+    `The value of "timeout" is out of range. It must be an unsigned integer. Received ${String(timeout)}`,
+  );
+  Reflect.set(error, "code", "ERR_OUT_OF_RANGE");
+  return error;
+};
+
 /** Captured subprocess output read from an execFileOutput rejection. */
 export interface ExecFileOutputFailure {
   readonly stdout: string;
@@ -93,6 +108,11 @@ export const execFileOutput = (
 ): Promise<{ readonly stdout: string; readonly stderr: string }> =>
   new Promise((resolve, reject) => {
     const { stopSignal, signal, timeout, ...execOptions } = options;
+    const invalid = timeout === undefined ? undefined : invalidTimeout(timeout);
+    if (invalid !== undefined) {
+      reject(invalid);
+      return;
+    }
     if (stopSignal !== undefined && signal?.aborted === true) {
       reject(abortError(signal));
       return;