new file mode 100644
--- /dev/null
+++ b/testing/web-platform/tests/streams/readable-streams/async-iterator.any.js
@@ -0,0 +1,340 @@
+// META: global=worker,jsshell
+// META: script=../resources/rs-utils.js
+// META: script=../resources/test-utils.js
+// META: script=../resources/recording-streams.js
+'use strict';
+
+test(() => {
+  assert_equals(ReadableStream.prototype[Symbol.asyncIterator], ReadableStream.prototype.getIterator);
+}, '@@asyncIterator() method is === to getIterator() method');
+
+test(() => {
+  const s = new ReadableStream();
+  const it = s.getIterator();
+  const proto = Object.getPrototypeOf(it);
+
+  const AsyncIteratorPrototype = Object.getPrototypeOf(Object.getPrototypeOf(async function* () {}).prototype);
+  assert_equals(Object.getPrototypeOf(proto), AsyncIteratorPrototype, 'prototype should extend AsyncIteratorPrototype');
+
+  const methods = ['next', 'return'].sort();
+  assert_array_equals(Object.getOwnPropertyNames(proto).sort(), methods, 'should have all the correct methods');
+
+  for (const m of methods) {
+    const propDesc = Object.getOwnPropertyDescriptor(proto, m);
+    assert_false(propDesc.enumerable, 'method should be non-enumerable');
+    assert_true(propDesc.configurable, 'method should be configurable');
+    assert_true(propDesc.writable, 'method should be writable');
+    assert_equals(typeof it[m], 'function', 'method should be a function');
+    assert_equals(it[m].name, m, 'method should have the correct name');
+  }
+
+  assert_equals(it.next.length, 0, 'next should have no parameters');
+  assert_equals(it.return.length, 1, 'return should have 1 parameter');
+  assert_equals(typeof it.throw, 'undefined', 'throw should not exist');
+}, 'Async iterator instances should have the correct list of properties');
+
+promise_test(async () => {
+  const s = new ReadableStream({
+    start(c) {
+      c.enqueue(1);
+      c.enqueue(2);
+      c.enqueue(3);
+      c.close();
+    },
+  });
+
+  const chunks = [];
+  for await (const chunk of s) {
+    chunks.push(chunk);
+  }
+  assert_array_equals(chunks, [1, 2, 3]);
+}, 'Async-iterating a push source');
+
+promise_test(async () => {
+  let i = 1;
+  const s = new ReadableStream({
+    pull(c) {
+      c.enqueue(i);
+      if (i >= 3) {
+        c.close();
+      }
+      i += 1;
+    },
+  });
+
+  const chunks = [];
+  for await (const chunk of s) {
+    chunks.push(chunk);
+  }
+  assert_array_equals(chunks, [1, 2, 3]);
+}, 'Async-iterating a pull source');
+
+promise_test(async () => {
+  let i = 1;
+  const s = recordingReadableStream({
+    pull(c) {
+      c.enqueue(i);
+      if (i >= 3) {
+        c.close();
+      }
+      i += 1;
+    },
+  }, new CountQueuingStrategy({ highWaterMark: 0 }));
+
+  const it = s.getIterator();
+  assert_array_equals(s.events, []);
+
+  const read1 = await it.next();
+  assert_equals(read1.done, false);
+  assert_equals(read1.value, 1);
+  assert_array_equals(s.events, ['pull']);
+
+  const read2 = await it.next();
+  assert_equals(read2.done, false);
+  assert_equals(read2.value, 2);
+  assert_array_equals(s.events, ['pull', 'pull']);
+
+  const read3 = await it.next();
+  assert_equals(read3.done, false);
+  assert_equals(read3.value, 3);
+  assert_array_equals(s.events, ['pull', 'pull', 'pull']);
+
+  const read4 = await it.next();
+  assert_equals(read4.done, true);
+  assert_equals(read4.value, undefined);
+  assert_array_equals(s.events, ['pull', 'pull', 'pull']);
+}, 'Async-iterating a pull source manually');
+
+promise_test(async () => {
+  const s = new ReadableStream({
+    start(c) {
+      c.error('e');
+    },
+  });
+
+  try {
+    for await (const chunk of s) {}
+    assert_unreached();
+  } catch (e) {
+    assert_equals(e, 'e');
+  }
+}, 'Async-iterating an errored stream throws');
+
+promise_test(async () => {
+  const s = new ReadableStream({
+    start(c) {
+      c.close();
+    }
+  });
+
+  for await (const chunk of s) {
+    assert_unreached();
+  }
+}, 'Async-iterating a closed stream never executes the loop body, but works fine');
+
+promise_test(async () => {
+  const s = new ReadableStream();
+
+  const loop = async () => {
+    for await (const chunk of s) {
+      assert_unreached();
+    }
+    assert_unreached();
+  };
+
+  await Promise.race([
+    loop(),
+    flushAsyncEvents()
+  ]);
+}, 'Async-iterating an empty but not closed/errored stream never executes the loop body and stalls the async function');
+
+promise_test(async () => {
+  const s = new ReadableStream({
+    start(c) {
+      c.enqueue(1);
+      c.enqueue(2);
+      c.enqueue(3);
+      c.close();
+    },
+  });
+
+  const reader = s.getReader();
+  const readResult = await reader.read();
+  assert_equals(readResult.done, false);
+  assert_equals(readResult.value, 1);
+  reader.releaseLock();
+
+  const chunks = [];
+  for await (const chunk of s) {
+    chunks.push(chunk);
+  }
+  assert_array_equals(chunks, [2, 3]);
+}, 'Async-iterating a partially consumed stream');
+
+for (const type of ['throw', 'break', 'return']) {
+  for (const preventCancel of [false, true]) {
+    promise_test(async () => {
+      const s = recordingReadableStream({
+        start(c) {
+          c.enqueue(0);
+        }
+      });
+
+      // use a separate function for the loop body so return does not stop the test
+      const loop = async () => {
+        for await (const c of s.getIterator({ preventCancel })) {
+          if (type === 'throw') {
+            throw new Error();
+          } else if (type === 'break') {
+            break;
+          } else if (type === 'return') {
+            return;
+          }
+        }
+      };
+
+      try {
+        await loop();
+      } catch (e) {}
+
+      if (preventCancel) {
+        assert_array_equals(s.events, ['pull'], `cancel() should not be called`);
+      } else {
+        assert_array_equals(s.events, ['pull', 'cancel', undefined], `cancel() should be called`);
+      }
+    }, `Cancellation behavior when ${type}ing inside loop body; preventCancel = ${preventCancel}`);
+  }
+}
+
+for (const preventCancel of [false, true]) {
+  promise_test(async () => {
+    const s = recordingReadableStream({
+      start(c) {
+        c.enqueue(0);
+      }
+    });
+
+    const it = s.getIterator({ preventCancel });
+    await it.return();
+
+    if (preventCancel) {
+      assert_array_equals(s.events, [], `cancel() should not be called`);
+    } else {
+      assert_array_equals(s.events, ['cancel', undefined], `cancel() should be called`);
+    }
+  }, `Cancellation behavior when manually calling return(); preventCancel = ${preventCancel}`);
+}
+
+promise_test(async () => {
+  const s = new ReadableStream();
+  const it = s[Symbol.asyncIterator]();
+  await it.return();
+  try {
+    await it.return();
+    assert_unreached();
+  } catch (e) {}
+}, 'Calling return() twice rejects');
+
+promise_test(async () => {
+  const s = new ReadableStream({
+    start(c) {
+      c.enqueue(0);
+      c.close();
+    },
+  });
+  const it = s[Symbol.asyncIterator]();
+  const next = await it.next();
+  assert_equals(Object.getPrototypeOf(next), Object.prototype);
+  assert_array_equals(Object.getOwnPropertyNames(next).sort(), ['done', 'value']);
+}, 'next()\'s fulfillment value has the right shape');
+
+promise_test(async t => {
+  const s = recordingReadableStream();
+  const it = s[Symbol.asyncIterator]();
+  it.next();
+
+  await promise_rejects(t, new TypeError(), it.return(), 'return() should reject');
+  assert_array_equals(s.events, ['pull']);
+}, 'calling return() while there are pending reads rejects');
+
+test(() => {
+  const s = new ReadableStream({
+    start(c) {
+      c.enqueue(0);
+      c.close();
+    },
+  });
+  const it = s.getIterator();
+  assert_throws(new TypeError(), () => s.getIterator(), 'getIterator() should throw');
+}, 'getIterator() throws if there\'s already a lock');
+
+promise_test(async () => {
+  const s = new ReadableStream({
+    start(c) {
+      c.enqueue(1);
+      c.enqueue(2);
+      c.enqueue(3);
+      c.close();
+    },
+  });
+
+  const chunks = [];
+  for await (const chunk of s) {
+    chunks.push(chunk);
+  }
+  assert_array_equals(chunks, [1, 2, 3]);
+
+  const reader = s.getReader();
+  await reader.closed;
+}, 'Acquiring a reader after exhaustively async-iterating a stream');
+
+promise_test(async () => {
+  const s = new ReadableStream({
+    start(c) {
+      c.enqueue(1);
+      c.enqueue(2);
+      c.enqueue(3);
+      c.close();
+    },
+  });
+
+  // read the first two chunks, then cancel
+  const chunks = [];
+  for await (const chunk of s) {
+    chunks.push(chunk);
+    if (chunk >= 2) {
+      break;
+    }
+  }
+  assert_array_equals(chunks, [1, 2]);
+
+  const reader = s.getReader();
+  await reader.closed;
+}, 'Acquiring a reader after partially async-iterating a stream');
+
+promise_test(async () => {
+  const s = new ReadableStream({
+    start(c) {
+      c.enqueue(1);
+      c.enqueue(2);
+      c.enqueue(3);
+      c.close();
+    },
+  });
+
+  // read the first two chunks, then release lock
+  const chunks = [];
+  for await (const chunk of s.getIterator({preventCancel: true})) {
+    chunks.push(chunk);
+    if (chunk >= 2) {
+      break;
+    }
+  }
+  assert_array_equals(chunks, [1, 2]);
+
+  const reader = s.getReader();
+  const readResult = await reader.read();
+  assert_equals(readResult.done, false, 'should not be closed yet');
+  assert_equals(readResult.value, 3, 'should read remaining chunk');
+  await reader.closed;
+}, 'Acquiring a reader and reading the remaining chunks after partially async-iterating a stream with preventCancel = true');

--- a/testing/web-platform/tests/streams/readable-streams/brand-checks.any.js
+++ b/testing/web-platform/tests/streams/readable-streams/brand-checks.any.js
@@ -1,14 +1,15 @@
 // META: global=worker,jsshell
 // META: script=../resources/test-utils.js
 'use strict';
 
 let ReadableStreamDefaultReader;
 let ReadableStreamDefaultController;
+let ReadableStreamAsyncIteratorPrototype;
 
 test(() => {
 
   // It's not exposed globally, but we test a few of its properties here.
   ReadableStreamDefaultReader = (new ReadableStream()).getReader().constructor;
 
 }, 'Can get the ReadableStreamDefaultReader constructor indirectly');
 
@@ -18,16 +19,23 @@ test(() => {
   new ReadableStream({
     start(c) {
       ReadableStreamDefaultController = c.constructor;
     }
   });
 
 }, 'Can get the ReadableStreamDefaultController constructor indirectly');
 
+test(() => {
+
+  const rs = new ReadableStream();
+  ReadableStreamAsyncIteratorPrototype = Object.getPrototypeOf(rs.getIterator());
+
+}, 'Can get ReadableStreamAsyncIteratorPrototype object indirectly');
+
 function fakeRS() {
   return Object.setPrototypeOf({
     cancel() { return Promise.resolve(); },
     getReader() { return new ReadableStreamDefaultReader(new ReadableStream()); },
     pipeThrough(obj) { return obj.readable; },
     pipeTo() { return Promise.resolve(); },
     tee() { return [realRS(), realRS()]; }
   }, ReadableStream.prototype);
@@ -63,16 +71,23 @@ function realRSDefaultController() {
   new ReadableStream({
     start(c) {
       controller = c;
     }
   });
   return controller;
 }
 
+function fakeRSAsyncIterator() {
+  return Object.setPrototypeOf({
+    next() { },
+    return(value = undefined) { }
+  }, ReadableStreamAsyncIteratorPrototype);
+}
+
 promise_test(t => {
 
   return methodRejectsForAll(t, ReadableStream.prototype, 'cancel',
                              [fakeRS(), realRSDefaultReader(), realRSDefaultController(), undefined, null]);
 
 }, 'ReadableStream.prototype.cancel enforces a brand check');
 
 test(() => {
@@ -152,8 +167,22 @@ test(() => {
 }, 'ReadableStreamDefaultController.prototype.enqueue enforces a brand check');
 
 test(() => {
 
   methodThrowsForAll(ReadableStreamDefaultController.prototype, 'error',
                      [fakeRSDefaultController(), realRS(), realRSDefaultReader(), undefined, null]);
 
 }, 'ReadableStreamDefaultController.prototype.error enforces a brand check');
+
+promise_test(t => {
+
+  return methodRejectsForAll(t, ReadableStreamAsyncIteratorPrototype, 'next',
+                             [fakeRSAsyncIterator(), realRS(), realRSDefaultReader(), undefined, null]);
+
+}, 'ReadableStreamAsyncIteratorPrototype.next enforces a brand check');
+
+promise_test(t => {
+
+  return methodRejectsForAll(t, ReadableStreamAsyncIteratorPrototype, 'return',
+                             [fakeRSAsyncIterator(), realRS(), realRSDefaultReader(), undefined, null]);
+
+}, 'ReadableStreamAsyncIteratorPrototype.return enforces a brand check');

--- a/testing/web-platform/tests/streams/readable-streams/general.any.js
+++ b/testing/web-platform/tests/streams/readable-streams/general.any.js
@@ -34,23 +34,25 @@ test(() => {
     'constructor should throw when the type is asdf');
   assert_throws(error1, () => new ReadableStream({ type: { get toString() {throw error1;} } }), 'constructor should throw when ToString() throws');
   assert_throws(error1, () => new ReadableStream({ type: { toString() {throw error1;} } }), 'constructor should throw when ToString() throws');
 
 }, 'ReadableStream can\'t be constructed with an invalid type');
 
 test(() => {
 
-  const methods = ['cancel', 'constructor', 'getReader', 'pipeThrough', 'pipeTo', 'tee'];
+  const methods = ['cancel', 'constructor', 'getReader', 'pipeThrough', 'pipeTo', 'tee', 'getIterator'];
   const properties = methods.concat(['locked']).sort();
+  const symbols = [Symbol.asyncIterator];
 
   const rs = new ReadableStream();
   const proto = Object.getPrototypeOf(rs);
 
-  assert_array_equals(Object.getOwnPropertyNames(proto).sort(), properties, 'should have all the correct methods');
+  assert_array_equals(Object.getOwnPropertyNames(proto).sort(), properties, 'should have all the correct properties');
+  assert_array_equals(Object.getOwnPropertySymbols(proto).sort(), symbols, 'should have all the correct symbols');
 
   for (const m of methods) {
     const propDesc = Object.getOwnPropertyDescriptor(proto, m);
     assert_false(propDesc.enumerable, 'method should be non-enumerable');
     assert_true(propDesc.configurable, 'method should be configurable');
     assert_true(propDesc.writable, 'method should be writable');
     assert_equals(typeof rs[m], 'function', 'method should be a function');
     const expectedName = m === 'constructor' ? 'ReadableStream' : m;
@@ -65,16 +67,25 @@ test(() => {
   assert_true(lockedPropDesc.configurable, 'locked should be configurable');
 
   assert_equals(rs.cancel.length, 1, 'cancel should have 1 parameter');
   assert_equals(rs.constructor.length, 0, 'constructor should have no parameters');
   assert_equals(rs.getReader.length, 0, 'getReader should have no parameters');
   assert_equals(rs.pipeThrough.length, 1, 'pipeThrough should have 1 parameters');
   assert_equals(rs.pipeTo.length, 1, 'pipeTo should have 1 parameter');
   assert_equals(rs.tee.length, 0, 'tee should have no parameters');
+  assert_equals(rs.getIterator.length, 0, 'getIterator should have no required parameters');
+  assert_equals(rs[Symbol.asyncIterator].length, 0, '@@asyncIterator should have no required parameters');
+
+  const asyncIteratorPropDesc = Object.getOwnPropertyDescriptor(proto, Symbol.asyncIterator);
+  assert_false(asyncIteratorPropDesc.enumerable, '@@asyncIterator should be non-enumerable');
+  assert_true(asyncIteratorPropDesc.configurable, '@@asyncIterator should be configurable');
+  assert_true(asyncIteratorPropDesc.writable, '@@asyncIterator should be writable');
+  assert_equals(typeof rs[Symbol.asyncIterator], 'function', '@@asyncIterator should be a function');
+  assert_equals(rs[Symbol.asyncIterator].name, 'getIterator', '@@asyncIterator should have the correct name');
 
 }, 'ReadableStream instances should have the correct list of properties');
 
 test(() => {
 
   assert_throws(new TypeError(), () => {
     new ReadableStream({ start: 'potato' });
   }, 'constructor should throw when start is not a function');

--- a/testing/web-platform/tests/streams/readable-streams/patched-global.any.js
+++ b/testing/web-platform/tests/streams/readable-streams/patched-global.any.js
@@ -52,8 +52,58 @@ test(t => {
     self.ReadableStream = oldReadableStream;
   });
 
   const [branch1, branch2] = rs.tee();
 
   assert_true(isReadableStream(branch1), 'branch1 should be a ReadableStream');
   assert_true(isReadableStream(branch2), 'branch2 should be a ReadableStream');
 }, 'ReadableStream tee() should not call the global ReadableStream');
+
+promise_test(async t => {
+  const rs = new ReadableStream({
+    start(c) {
+      c.enqueue(1);
+      c.enqueue(2);
+      c.enqueue(3);
+      c.close();
+    }
+  });
+
+  const oldReadableStreamGetReader = ReadableStream.prototype.getReader;
+
+  const ReadableStreamDefaultReader = (new ReadableStream()).getReader().constructor;
+  const oldDefaultReaderRead = ReadableStreamDefaultReader.prototype.read;
+  const oldDefaultReaderCancel = ReadableStreamDefaultReader.prototype.cancel;
+  const oldDefaultReaderReleaseLock = ReadableStreamDefaultReader.prototype.releaseLock;
+
+  self.ReadableStream.prototype.getReader = function() {
+    throw new Error('patched getReader() called');
+  };
+
+  ReadableStreamDefaultReader.prototype.read = function() {
+    throw new Error('patched read() called');
+  };
+  ReadableStreamDefaultReader.prototype.cancel = function() {
+    throw new Error('patched cancel() called');
+  };
+  ReadableStreamDefaultReader.prototype.releaseLock = function() {
+    throw new Error('patched releaseLock() called');
+  };
+
+  t.add_cleanup(() => {
+    self.ReadableStream.prototype.getReader = oldReadableStreamGetReader;
+
+    ReadableStreamDefaultReader.prototype.read = oldDefaultReaderRead;
+    ReadableStreamDefaultReader.prototype.cancel = oldDefaultReaderCancel;
+    ReadableStreamDefaultReader.prototype.releaseLock = oldDefaultReaderReleaseLock;
+  });
+
+  // read the first chunk, then cancel
+  for await (const chunk of rs) {
+    break;
+  }
+
+  // should be able to acquire a new reader
+  const reader = oldReadableStreamGetReader.call(rs);
+  // stream should be cancelled
+  await reader.closed;
+}, 'ReadableStream getIterator() should use the original values of getReader() and ReadableStreamDefaultReader methods');