diff --git a/packages/deno/src/utils/streaming.ts b/packages/deno/src/utils/streaming.ts
index 045a104c5e93..35233f9b1a5f 100644
--- a/packages/deno/src/utils/streaming.ts
+++ b/packages/deno/src/utils/streaming.ts
@@ -81,8 +81,10 @@ function monitorStream(
   onDone: () => void,
 ): ReadableStream<Uint8Array<ArrayBufferLike>> {
   const reader = stream.getReader();
-  // oxlint-disable-next-line typescript/no-floating-promises
-  reader.closed.finally(() => onDone());
+  reader.closed.then(
+    () => onDone(),
+    () => onDone(),
+  );
   return new ReadableStream({
     async start(controller) {
       let result: ReadableStreamReadResult<Uint8Array<ArrayBufferLike>>;
diff --git a/packages/deno/test/streaming.test.ts b/packages/deno/test/streaming.test.ts
new file mode 100644
index 000000000000..d9849ece3c76
--- /dev/null
+++ b/packages/deno/test/streaming.test.ts
@@ -0,0 +1,56 @@
+// <reference lib="deno.ns" />
+
+import { assertEquals } from 'https://deno.land/std@0.212.0/assert/mod.ts';
+
+Deno.test('reader.closed.then(f, f) suppresses rejection when releaseLock is called on an open stream', async () => {
+  // Reproduces the bug from GitHub issue #20177:
+  // In monitorStream, reader.releaseLock() is called while the source stream
+  // is still open (e.g. the error path when controller.enqueue() throws).
+  // Per WHATWG Streams spec, this rejects reader.closed with a TypeError.
+  // Using .then(onDone, onDone) handles both cases; .finally() would propagate
+  // the rejection as unhandled.
+
+  let onDoneCalled = false;
+
+  const stream = new ReadableStream<Uint8Array>({
+    start(controller) {
+      controller.enqueue(new TextEncoder().encode('data'));
+      // intentionally not closing — stream stays open
+    },
+  });
+
+  const reader = stream.getReader();
+
+  // This is the exact pattern from monitorStream (line 84 in streaming.ts).
+  // With .finally(() => onDone()), this would propagate the rejection.
+  reader.closed.then(
+    () => {
+      onDoneCalled = true;
+    },
+    () => {
+      onDoneCalled = true;
+    },
+  );
+
+  await reader.read();
+
+  // This is what monitorStream does on the error path (line 98) when
+  // controller.enqueue() throws — releaseLock while the source is still open.
+  reader.releaseLock();
+
+  let unhandledRejection: PromiseRejectionEvent | undefined;
+  const handler = (e: PromiseRejectionEvent): void => {
+    e.preventDefault();
+    unhandledRejection = e;
+  };
+  globalThis.addEventListener('unhandledrejection', handler);
+
+  try {
+    await new Promise(resolve => setTimeout(resolve, 50));
+
+    assertEquals(onDoneCalled, true, 'onDone should have been called via the rejection handler');
+    assertEquals(unhandledRejection, undefined, 'should not have caused an unhandled promise rejection');
+  } finally {
+    globalThis.removeEventListener('unhandledrejection', handler);
+  }
+});