diff --git a/testing/web-platform/tests/native-file-system/NativeFileSystemWritableFileStream.tentative.window.js b/testing/web-platform/tests/native-file-system/NativeFileSystemWritableFileStream.tentative.window.js
new file mode 100644
index 0000000000000..1dbcb2b136220
--- /dev/null
+++ b/testing/web-platform/tests/native-file-system/NativeFileSystemWritableFileStream.tentative.window.js
@@ -0,0 +1,126 @@
+// META: script=resources/test-helpers.js
+promise_test(async t => cleanupSandboxedFileSystem(),
+             'Cleanup to setup test environment');
+
+promise_test(async t => {
+  const handle = await createEmptyFile(t, 'empty_blob');
+  const stream = await handle.createWritable();
+
+  await stream.write(0, new Blob([]));
+
+  assert_equals(await getFileContents(handle), '');
+  assert_equals(await getFileSize(handle), 0);
+}, 'write() with an empty blob to an empty file');
+
+promise_test(async t => {
+  const handle = await createEmptyFile(t, 'valid_blob');
+  const stream = await handle.createWritable();
+
+  await stream.write(0, new Blob(['1234567890']));
+
+  assert_equals(await getFileContents(handle), '1234567890');
+  assert_equals(await getFileSize(handle), 10);
+}, 'write() a blob to an empty file');
+
+promise_test(async t => {
+    const handle = await createEmptyFile(t, 'blob_with_offset');
+    const stream = await handle.createWritable();
+
+    await stream.write(0, new Blob(['1234567890']));
+    await stream.write(4, new Blob(['abc']));
+
+    assert_equals(await getFileContents(handle), '1234abc890');
+    assert_equals(await getFileSize(handle), 10);
+}, 'write() called with a blob and a valid offset');
+
+promise_test(async t => {
+    const handle = await createEmptyFile(t, 'bad_offset');
+    const stream = await handle.createWritable();
+
+    await promise_rejects(t, 'InvalidStateError', stream.write(4, new Blob(['abc'])));
+
+    assert_equals(await getFileContents(handle), '');
+    assert_equals(await getFileSize(handle), 0);
+}, 'write() called with an invalid offset');
+
+promise_test(async t => {
+  const handle = await createEmptyFile(t, 'empty_string');
+  const stream = await handle.createWritable();
+
+  await stream.write(0, '');
+  assert_equals(await getFileContents(handle), '');
+  assert_equals(await getFileSize(handle), 0);
+}, 'write() with an empty string to an empty file');
+
+promise_test(async t => {
+  const handle = await createEmptyFile(t, 'valid_utf8_string');
+  const stream = await handle.createWritable();
+
+  await stream.write(0, 'foo🤘');
+  assert_equals(await getFileContents(handle), 'foo🤘');
+  assert_equals(await getFileSize(handle), 7);
+}, 'write() with a valid utf-8 string');
+
+promise_test(async t => {
+  const handle = await createEmptyFile(t, 'string_with_unix_line_ending');
+  const stream = await handle.createWritable();
+
+  await stream.write(0, 'foo\n');
+  assert_equals(await getFileContents(handle), 'foo\n');
+  assert_equals(await getFileSize(handle), 4);
+}, 'write() with a string with unix line ending preserved');
+
+promise_test(async t => {
+  const handle = await createEmptyFile(t, 'string_with_windows_line_ending');
+  const stream = await handle.createWritable();
+
+  await stream.write(0, 'foo\r\n');
+  assert_equals(await getFileContents(handle), 'foo\r\n');
+  assert_equals(await getFileSize(handle), 5);
+}, 'write() with a string with windows line ending preserved');
+
+promise_test(async t => {
+  const handle = await createEmptyFile(t, 'empty_array_buffer');
+  const stream = await handle.createWritable();
+
+  let buf = new ArrayBuffer(0);
+  await stream.write(0, buf);
+  assert_equals(await getFileContents(handle), '');
+  assert_equals(await getFileSize(handle), 0);
+}, 'write() with an empty array buffer to an empty file');
+
+promise_test(async t => {
+  const handle = await createEmptyFile(t, 'valid_string_typed_byte_array');
+  const stream = await handle.createWritable();
+
+  let buf = new ArrayBuffer(3);
+  let intView = new Uint8Array(buf);
+  intView[0] = 0x66;
+  intView[1] = 0x6f;
+  intView[2] = 0x6f;
+  await stream.write(0, buf);
+  assert_equals(await getFileContents(handle), 'foo');
+  assert_equals(await getFileSize(handle), 3);
+}, 'write() with a valid typed array buffer');
+
+promise_test(async t => {
+    const handle = await createEmptyFile(t, 'trunc_shrink');
+    const stream = await handle.createWritable();
+
+    await stream.write(0, new Blob(['1234567890']));
+    await stream.truncate(5);
+
+    assert_equals(await getFileContents(handle), '12345');
+    assert_equals(await getFileSize(handle), 5);
+}, 'truncate() to shrink a file');
+
+promise_test(async t => {
+    const handle = await createEmptyFile(t, 'trunc_grow');
+    const stream = await handle.createWritable();
+
+    await stream.write(0, new Blob(['abc']));
+    await stream.truncate(5);
+
+    assert_equals(await getFileContents(handle), 'abc\0\0');
+    assert_equals(await getFileSize(handle), 5);
+}, 'truncate() to grow a file');