diff --git a/ports/webassembly/Makefile b/ports/webassembly/Makefile
index d0a8aa9924..93b92ef583 100644
--- a/ports/webassembly/Makefile
+++ b/ports/webassembly/Makefile
@@ -22,6 +22,9 @@ BUILD ?= build-$(VARIANT)
 include ../../py/mkenv.mk
 include $(VARIANT_DIR)/mpconfigvariant.mk
 
+# Use the default frozen manifest, variants may override this.
+FROZEN_MANIFEST ?= variants/manifest.py
+
 # Qstr definitions (must come before including py.mk).
 QSTR_DEFS = qstrdefsport.h
 
diff --git a/ports/webassembly/asyncio/__init__.py b/ports/webassembly/asyncio/__init__.py
new file mode 100644
index 0000000000..ba1ca63514
--- /dev/null
+++ b/ports/webassembly/asyncio/__init__.py
@@ -0,0 +1,9 @@
+# MicroPython asyncio module, for use with webassembly port
+# MIT license; Copyright (c) 2024 Damien P. George
+
+from .core import *
+from .funcs import wait_for, wait_for_ms, gather
+from .event import Event
+from .lock import Lock
+
+__version__ = (3, 0, 0)
diff --git a/ports/webassembly/asyncio/core.py b/ports/webassembly/asyncio/core.py
new file mode 100644
index 0000000000..a128bb6055
--- /dev/null
+++ b/ports/webassembly/asyncio/core.py
@@ -0,0 +1,249 @@
+# MicroPython asyncio module, for use with webassembly port
+# MIT license; Copyright (c) 2019-2024 Damien P. George
+
+from time import ticks_ms as ticks, ticks_diff, ticks_add
+import sys, js, jsffi
+
+# Import TaskQueue and Task from built-in C code.
+from _asyncio import TaskQueue, Task
+
+
+################################################################################
+# Exceptions
+
+
+class CancelledError(BaseException):
+    pass
+
+
+class TimeoutError(Exception):
+    pass
+
+
+# Used when calling Loop.call_exception_handler.
+_exc_context = {"message": "Task exception wasn't retrieved", "exception": None, "future": None}
+
+
+################################################################################
+# Sleep functions
+
+
+# "Yield" once, then raise StopIteration
+class SingletonGenerator:
+    def __init__(self):
+        self.state = None
+        self.exc = StopIteration()
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        if self.state is not None:
+            _task_queue.push(cur_task, self.state)
+            self.state = None
+            return None
+        else:
+            self.exc.__traceback__ = None
+            raise self.exc
+
+
+# Pause task execution for the given time (integer in milliseconds, uPy extension)
+# Use a SingletonGenerator to do it without allocating on the heap
+def sleep_ms(t, sgen=SingletonGenerator()):
+    if cur_task is None:
+        # Support top-level asyncio.sleep, via a JavaScript Promise.
+        return jsffi.async_timeout_ms(t)
+    assert sgen.state is None
+    sgen.state = ticks_add(ticks(), max(0, t))
+    return sgen
+
+
+# Pause task execution for the given time (in seconds)
+def sleep(t):
+    return sleep_ms(int(t * 1000))
+
+
+################################################################################
+# Main run loop
+
+asyncio_timer = None
+
+
+class ThenableEvent:
+    def __init__(self, thenable):
+        self.result = None  # Result of the thenable
+        self.waiting = None  # Task waiting on completion of this thenable
+        thenable.then(self.set)
+
+    def set(self, value):
+        # Thenable/Promise is fulfilled, set result and schedule any waiting task.
+        self.result = value
+        if self.waiting:
+            _task_queue.push(self.waiting)
+            self.waiting = None
+            _schedule_run_iter(0)
+
+    def remove(self, task):
+        self.waiting = None
+
+    # async
+    def wait(self):
+        # Set the calling task as the task waiting on this thenable.
+        self.waiting = cur_task
+        # Set calling task's data to this object so it can be removed if needed.
+        cur_task.data = self
+        # Wait for the thenable to fulfill.
+        yield
+        # Return the result of the thenable.
+        return self.result
+
+
+# Ensure the awaitable is a task
+def _promote_to_task(aw):
+    return aw if isinstance(aw, Task) else create_task(aw)
+
+
+def _schedule_run_iter(dt):
+    global asyncio_timer
+    if asyncio_timer is not None:
+        js.clearTimeout(asyncio_timer)
+    asyncio_timer = js.setTimeout(_run_iter, dt)
+
+
+def _run_iter():
+    global cur_task
+    excs_all = (CancelledError, Exception)  # To prevent heap allocation in loop
+    excs_stop = (CancelledError, StopIteration)  # To prevent heap allocation in loop
+    while True:
+        # Wait until the head of _task_queue is ready to run
+        t = _task_queue.peek()
+        if t:
+            # A task waiting on _task_queue; "ph_key" is time to schedule task at
+            dt = max(0, ticks_diff(t.ph_key, ticks()))
+        else:
+            # No tasks can be woken so finished running
+            cur_task = None
+            return
+
+        if dt > 0:
+            # schedule to call again later
+            cur_task = None
+            _schedule_run_iter(dt)
+            return
+
+        # Get next task to run and continue it
+        t = _task_queue.pop()
+        cur_task = t
+        try:
+            # Continue running the coroutine, it's responsible for rescheduling itself
+            exc = t.data
+            if not exc:
+                t.coro.send(None)
+            else:
+                # If the task is finished and on the run queue and gets here, then it
+                # had an exception and was not await'ed on.  Throwing into it now will
+                # raise StopIteration and the code below will catch this and run the
+                # call_exception_handler function.
+                t.data = None
+                t.coro.throw(exc)
+        except excs_all as er:
+            # Check the task is not on any event queue
+            assert t.data is None
+            # This task is done.
+            if t.state:
+                # Task was running but is now finished.
+                waiting = False
+                if t.state is True:
+                    # "None" indicates that the task is complete and not await'ed on (yet).
+                    t.state = None
+                elif callable(t.state):
+                    # The task has a callback registered to be called on completion.
+                    t.state(t, er)
+                    t.state = False
+                    waiting = True
+                else:
+                    # Schedule any other tasks waiting on the completion of this task.
+                    while t.state.peek():
+                        _task_queue.push(t.state.pop())
+                        waiting = True
+                    # "False" indicates that the task is complete and has been await'ed on.
+                    t.state = False
+                if not waiting and not isinstance(er, excs_stop):
+                    # An exception ended this detached task, so queue it for later
+                    # execution to handle the uncaught exception if no other task retrieves
+                    # the exception in the meantime (this is handled by Task.throw).
+                    _task_queue.push(t)
+                # Save return value of coro to pass up to caller.
+                t.data = er
+            elif t.state is None:
+                # Task is already finished and nothing await'ed on the task,
+                # so call the exception handler.
+
+                # Save exception raised by the coro for later use.
+                t.data = exc
+
+                # Create exception context and call the exception handler.
+                _exc_context["exception"] = exc
+                _exc_context["future"] = t
+                Loop.call_exception_handler(_exc_context)
+
+
+# Create and schedule a new task from a coroutine.
+def create_task(coro):
+    if not hasattr(coro, "send"):
+        raise TypeError("coroutine expected")
+    t = Task(coro, globals())
+    _task_queue.push(t)
+    _schedule_run_iter(0)
+    return t
+
+
+################################################################################
+# Event loop wrapper
+
+
+cur_task = None
+
+
+class Loop:
+    _exc_handler = None
+
+    def create_task(coro):
+        return create_task(coro)
+
+    def close():
+        pass
+
+    def set_exception_handler(handler):
+        Loop._exc_handler = handler
+
+    def get_exception_handler():
+        return Loop._exc_handler
+
+    def default_exception_handler(loop, context):
+        print(context["message"], file=sys.stderr)
+        print("future:", context["future"], "coro=", context["future"].coro, file=sys.stderr)
+        sys.print_exception(context["exception"], sys.stderr)
+
+    def call_exception_handler(context):
+        (Loop._exc_handler or Loop.default_exception_handler)(Loop, context)
+
+
+def get_event_loop():
+    return Loop
+
+
+def current_task():
+    if cur_task is None:
+        raise RuntimeError("no running event loop")
+    return cur_task
+
+
+def new_event_loop():
+    global _task_queue
+    _task_queue = TaskQueue()  # TaskQueue of Task instances.
+    return Loop
+
+
+# Initialise default event loop.
+new_event_loop()
diff --git a/ports/webassembly/objjsproxy.c b/ports/webassembly/objjsproxy.c
index 65c806536d..15fbb57523 100644
--- a/ports/webassembly/objjsproxy.c
+++ b/ports/webassembly/objjsproxy.c
@@ -474,9 +474,29 @@ static mp_obj_t jsproxy_new_gen(mp_obj_t self_in, mp_obj_iter_buf_t *iter_buf) {
 
 /******************************************************************************/
 
+#if MICROPY_PY_ASYNCIO
+extern mp_obj_t mp_asyncio_context;
+#endif
+
 static mp_obj_t jsproxy_getiter(mp_obj_t self_in, mp_obj_iter_buf_t *iter_buf) {
     mp_obj_jsproxy_t *self = MP_OBJ_TO_PTR(self_in);
     if (has_attr(self->ref, "then")) {
+        #if MICROPY_PY_ASYNCIO
+        // When asyncio is running and the caller here is a task, wrap the JavaScript
+        // thenable in a ThenableEvent, and get the task to wait on that event.  This
+        // decouples the task from the thenable and allows cancelling the task.
+        if (mp_asyncio_context != MP_OBJ_NULL) {
+            mp_obj_t cur_task = mp_obj_dict_get(mp_asyncio_context, MP_OBJ_NEW_QSTR(MP_QSTR_cur_task));
+            if (cur_task != mp_const_none) {
+                mp_obj_t thenable_event_class = mp_obj_dict_get(mp_asyncio_context, MP_OBJ_NEW_QSTR(MP_QSTR_ThenableEvent));
+                mp_obj_t thenable_event = mp_call_function_1(thenable_event_class, self_in);
+                mp_obj_t dest[2];
+                mp_load_method(thenable_event, MP_QSTR_wait, dest);
+                mp_obj_t wait_gen = mp_call_method_n_kw(0, 0, dest);
+                return mp_getiter(wait_gen, iter_buf);
+            }
+        }
+        #endif
         return jsproxy_new_gen(self_in, iter_buf);
     } else {
         return jsproxy_new_it(self_in, iter_buf);
diff --git a/ports/webassembly/variants/manifest.py b/ports/webassembly/variants/manifest.py
new file mode 100644
index 0000000000..b2ee8cd640
--- /dev/null
+++ b/ports/webassembly/variants/manifest.py
@@ -0,0 +1,24 @@
+# The asyncio package is built from the standard implementation but with the
+# core scheduler replaced with a custom scheduler that uses the JavaScript
+# runtime (with setTimeout an Promise's) to contrtol the scheduling.
+
+package(
+    "asyncio",
+    (
+        "event.py",
+        "funcs.py",
+        "lock.py",
+    ),
+    base_path="$(MPY_DIR)/extmod",
+    opt=3,
+)
+
+package(
+    "asyncio",
+    (
+        "__init__.py",
+        "core.py",
+    ),
+    base_path="$(PORT_DIR)",
+    opt=3,
+)
diff --git a/ports/webassembly/variants/pyscript/manifest.py b/ports/webassembly/variants/pyscript/manifest.py
index 0646e1d897..db088e70d0 100644
--- a/ports/webassembly/variants/pyscript/manifest.py
+++ b/ports/webassembly/variants/pyscript/manifest.py
@@ -1,3 +1,5 @@
+include("$(PORT_DIR)/variants/manifest.py")
+
 require("abc")
 require("base64")
 require("collections")
diff --git a/tests/ports/webassembly/asyncio_create_task.mjs b/tests/ports/webassembly/asyncio_create_task.mjs
new file mode 100644
index 0000000000..e388ade4f1
--- /dev/null
+++ b/tests/ports/webassembly/asyncio_create_task.mjs
@@ -0,0 +1,44 @@
+// Test asyncio.create_task(), and tasks waiting on a Promise.
+
+const mp = await (await import(process.argv[2])).loadMicroPython();
+
+globalThis.p0 = new Promise((resolve, reject) => {
+    resolve(123);
+});
+
+globalThis.p1 = new Promise((resolve, reject) => {
+    setTimeout(() => {
+        console.log("setTimeout resolved");
+        resolve(456);
+    }, 200);
+});
+
+mp.runPython(`
+import js
+import asyncio
+
+async def task(id, promise):
+    print("task start", id)
+    print("task await", id, await promise)
+    print("task await", id, await promise)
+    print("task end", id)
+
+print("start")
+t1 = asyncio.create_task(task(1, js.p0))
+t2 = asyncio.create_task(task(2, js.p1))
+print("t1", t1.done(), t2.done())
+print("end")
+`);
+
+// Wait for p1 to fulfill so t2 can continue.
+await globalThis.p1;
+
+// Wait a little longer so t2 can complete.
+await new Promise((resolve, reject) => {
+    setTimeout(resolve, 10);
+});
+
+mp.runPython(`
+print("restart")
+print("t1", t1.done(), t2.done())
+`);
diff --git a/tests/ports/webassembly/asyncio_create_task.mjs.exp b/tests/ports/webassembly/asyncio_create_task.mjs.exp
new file mode 100644
index 0000000000..c1958bba59
--- /dev/null
+++ b/tests/ports/webassembly/asyncio_create_task.mjs.exp
@@ -0,0 +1,14 @@
+start
+t1 False False
+end
+task start 1
+task start 2
+task await 1 123
+task await 1 123
+task end 1
+setTimeout resolved
+task await 2 456
+task await 2 456
+task end 2
+restart
+t1 True True
diff --git a/tests/ports/webassembly/asyncio_sleep.mjs b/tests/ports/webassembly/asyncio_sleep.mjs
new file mode 100644
index 0000000000..74d22ee1f8
--- /dev/null
+++ b/tests/ports/webassembly/asyncio_sleep.mjs
@@ -0,0 +1,25 @@
+// Test asyncio.sleep(), both at the top level and within a task.
+
+const mp = await (await import(process.argv[2])).loadMicroPython();
+
+await mp.runPythonAsync(`
+import time
+import asyncio
+
+print("main start")
+t0 = time.time()
+await asyncio.sleep(0.25)
+dt = time.time() - t0
+print(0.2 <= dt <= 0.3)
+
+async def task():
+    print("task start")
+    t0 = time.time()
+    await asyncio.sleep(0.25)
+    dt = time.time() - t0
+    print(0.2 <= dt <= 0.3)
+    print("task end")
+
+asyncio.create_task(task())
+print("main end")
+`);
diff --git a/tests/ports/webassembly/asyncio_sleep.mjs.exp b/tests/ports/webassembly/asyncio_sleep.mjs.exp
new file mode 100644
index 0000000000..619ba175f6
--- /dev/null
+++ b/tests/ports/webassembly/asyncio_sleep.mjs.exp
@@ -0,0 +1,6 @@
+main start
+True
+main end
+task start
+True
+task end
diff --git a/tests/run-tests.py b/tests/run-tests.py
index 4f55cdd398..8acdcd2b36 100755
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -681,6 +681,17 @@ def run_tests(pyb, tests, args, result_dir, num_threads=1):
         elif args.target == "webassembly":
             skip_tests.add("basics/string_format_modulo.py")  # can't print nulls to stdout
             skip_tests.add("basics/string_strip.py")  # can't print nulls to stdout
+            skip_tests.add("extmod/asyncio_basic2.py")
+            skip_tests.add("extmod/asyncio_cancel_self.py")
+            skip_tests.add("extmod/asyncio_current_task.py")
+            skip_tests.add("extmod/asyncio_exception.py")
+            skip_tests.add("extmod/asyncio_gather_finished_early.py")
+            skip_tests.add("extmod/asyncio_get_event_loop.py")
+            skip_tests.add("extmod/asyncio_heaplock.py")
+            skip_tests.add("extmod/asyncio_loop_stop.py")
+            skip_tests.add("extmod/asyncio_new_event_loop.py")
+            skip_tests.add("extmod/asyncio_threadsafeflag.py")
+            skip_tests.add("extmod/asyncio_wait_for_fwd.py")
             skip_tests.add("extmod/binascii_a2b_base64.py")
             skip_tests.add("extmod/re_stack_overflow.py")
             skip_tests.add("extmod/time_res.py")