diff --git a/Cargo.toml b/Cargo.toml
index c1c756a6..b8b1eab5 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -30,6 +30,7 @@ libp2p-core = { version = "0.7.0", path = "./core" }
 libp2p-core-derive = { version = "0.7.0", path = "./misc/core-derive" }
 libp2p-secio = { version = "0.7.0", path = "./protocols/secio", default-features = false }
 libp2p-uds = { version = "0.7.0", path = "./transports/uds" }
+libp2p-wasm-ext = { version = "0.1.0", path = "./transports/wasm-ext" }
 libp2p-websocket = { version = "0.7.0", path = "./transports/websocket", optional = true }
 libp2p-yamux = { version = "0.7.0", path = "./muxers/yamux" }
 parking_lot = "0.7"
@@ -76,5 +77,6 @@ members = [
     "transports/ratelimit",
     "transports/tcp",
     "transports/uds",
-    "transports/websocket"
+    "transports/websocket",
+    "transports/wasm-ext"
 ]
diff --git a/transports/wasm-ext/Cargo.toml b/transports/wasm-ext/Cargo.toml
new file mode 100644
index 00000000..141c9ab1
--- /dev/null
+++ b/transports/wasm-ext/Cargo.toml
@@ -0,0 +1,19 @@
+[package]
+name = "libp2p-wasm-ext"
+version = "0.1.0"
+authors = ["Pierre Krieger <pierre.krieger1708@gmail.com>"]
+edition = "2018"
+description = "Allows passing in an external transport in a WASM environment"
+license = "MIT"
+repository = "https://github.com/libp2p/rust-libp2p"
+keywords = ["peer-to-peer", "libp2p", "networking"]
+categories = ["network-programming", "asynchronous"]
+
+[dependencies]
+futures = "0.1"
+js-sys = "0.3.19"
+libp2p-core = { version = "0.7.0", path = "../../core" }
+send_wrapper = "0.2.0"
+tokio-io = "0.1"
+wasm-bindgen = "0.2.42"
+wasm-bindgen-futures = "0.3.19"
diff --git a/transports/wasm-ext/src/lib.rs b/transports/wasm-ext/src/lib.rs
new file mode 100644
index 00000000..8577590d
--- /dev/null
+++ b/transports/wasm-ext/src/lib.rs
@@ -0,0 +1,518 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+//! Implementation of the libp2p `Transport` trait for external transports.
+//!
+//! This `Transport` is used in the context of WASM to allow delegating the transport mechanism
+//! to the code that uses rust-libp2p, as opposed to inside of rust-libp2p itself.
+//!
+//! > **Note**: This only allows transports that produce a raw stream with the remote. You
+//! >           couldn't, for example, pass an implementation QUIC.
+//!
+//! # Usage
+//!
+//! Call `new()` with a JavaScript object that implements the interface described in the `ffi`
+//! module.
+//!
+
+use futures::{future::FutureResult, prelude::*, stream::Stream, try_ready};
+use libp2p_core::{transport::ListenerEvent, transport::TransportError, Multiaddr, Transport};
+use send_wrapper::SendWrapper;
+use std::{collections::VecDeque, error, fmt, io, mem};
+use wasm_bindgen::{JsCast, prelude::*};
+
+/// Contains the definition that one must match on the JavaScript side.
+pub mod ffi {
+    use wasm_bindgen::prelude::*;
+
+    #[wasm_bindgen]
+    extern "C" {
+        /// Type of the object that allows opening connections.
+        pub type Transport;
+        /// Type of the object that represents an open connection with a remote.
+        pub type Connection;
+        /// Type of the object that represents an event generated by listening.
+        pub type ListenEvent;
+        /// Type of the object that represents an event containing a new connection with a remote.
+        pub type ConnectionEvent;
+
+        /// Start attempting to dial the given multiaddress.
+        ///
+        /// The returned `Promise` must yield a [`Connection`] on success.
+        ///
+        /// If the multiaddress is not supported, you should return an instance of `Error` whose
+        /// `name` property has been set to the string `"NotSupportedError"`.
+        #[wasm_bindgen(method, catch)]
+        pub fn dial(this: &Transport, multiaddr: &str) -> Result<js_sys::Promise, JsValue>;
+
+        /// Start listening on the given multiaddress.
+        ///
+        /// The returned `Iterator` must yield `Promise`s to [`ListenEvent`] events.
+        ///
+        /// If the multiaddress is not supported, you should return an instance of `Error` whose
+        /// `name` property has been set to the string `"NotSupportedError"`.
+        #[wasm_bindgen(method, catch)]
+        pub fn listen_on(this: &Transport, multiaddr: &str) -> Result<js_sys::Iterator, JsValue>;
+
+        /// Returns a `Readable​Stream​` that .
+        #[wasm_bindgen(method, getter)]
+        pub fn read(this: &Connection) -> js_sys::Iterator;
+
+        /// Writes data to the connection. Returns a `Promise` that resolves when the connection is
+        /// ready for writing again.
+        ///
+        /// If the `Promise` returns an error, the writing side of the connection is considered
+        /// unrecoverable and the connection should be closed as soon as possible.
+        ///
+        /// Guaranteed to only be called after the previous write promise has resolved.
+        #[wasm_bindgen(method, catch)]
+        pub fn write(this: &Connection, data: &[u8]) -> Result<js_sys::Promise, JsValue>;
+
+        /// Shuts down the writing side of the connection. After this has been called, the `write`
+        /// method will no longer be called.
+        #[wasm_bindgen(method, catch)]
+        pub fn shutdown(this: &Connection) -> Result<(), JsValue>;
+
+        /// Closes the connection. No other method will be called on this connection anymore.
+        #[wasm_bindgen(method)]
+        pub fn close(this: &Connection);
+
+        /// List of addresses we have started listening on. Must be an array of strings of multiaddrs.
+        #[wasm_bindgen(method, getter)]
+        pub fn new_addrs(this: &ListenEvent) -> Option<Box<[JsValue]>>;
+
+        /// List of addresses that have expired. Must be an array of strings of multiaddrs.
+        #[wasm_bindgen(method, getter)]
+        pub fn expired_addrs(this: &ListenEvent) -> Option<Box<[JsValue]>>;
+
+        /// List of [`ConnectionEvent`] object that has been received.
+        #[wasm_bindgen(method, getter)]
+        pub fn new_connections(this: &ListenEvent) -> Option<Box<[JsValue]>>;
+
+        /// Promise to the next event that the listener will generate.
+        #[wasm_bindgen(method, getter)]
+        pub fn next_event(this: &ListenEvent) -> JsValue;
+
+        /// The [`Connection`] object for communication with the remote.
+        #[wasm_bindgen(method, getter)]
+        pub fn connection(this: &ConnectionEvent) -> Connection;
+
+        /// The address we observe for the remote connection.
+        #[wasm_bindgen(method, getter)]
+        pub fn observed_addr(this: &ConnectionEvent) -> String;
+
+        /// The address we are listening on, that received the remote connection.
+        #[wasm_bindgen(method, getter)]
+        pub fn local_addr(this: &ConnectionEvent) -> String;
+    }
+}
+
+/// Implementation of `Transport` whose implementation is handled by some FFI.
+pub struct ExtTransport {
+    inner: SendWrapper<ffi::Transport>,
+}
+
+impl ExtTransport {
+    /// Creates a new `ExtTransport` that uses the given external `Transport`.
+    pub fn new(transport: ffi::Transport) -> Self {
+        ExtTransport {
+            inner: SendWrapper::new(transport),
+        }
+    }
+}
+
+impl fmt::Debug for ExtTransport {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_tuple("ExtTransport").finish()
+    }
+}
+
+impl Clone for ExtTransport {
+    fn clone(&self) -> Self {
+        ExtTransport {
+            inner: SendWrapper::new(self.inner.clone().into()),
+        }
+    }
+}
+
+impl Transport for ExtTransport {
+    type Output = Connection;
+    type Error = JsErr;
+    type Listener = Listen;
+    type ListenerUpgrade = FutureResult<Self::Output, Self::Error>;
+    type Dial = Dial;
+
+    fn listen_on(self, addr: Multiaddr) -> Result<Self::Listener, TransportError<Self::Error>> {
+        let iter = self
+            .inner
+            .listen_on(&addr.to_string())
+            .map_err(|err| {
+                if is_not_supported_error(&err) {
+                    TransportError::MultiaddrNotSupported(addr)
+                } else {
+                    TransportError::Other(JsErr::from(err))
+                }
+            })?;
+
+        Ok(Listen {
+            iterator: SendWrapper::new(iter),
+            next_event: None,
+            pending_events: VecDeque::new(),
+        })
+    }
+
+    fn dial(self, addr: Multiaddr) -> Result<Self::Dial, TransportError<Self::Error>> {
+        let promise = self
+            .inner
+            .dial(&addr.to_string())
+            .map_err(|err| {
+                if is_not_supported_error(&err) {
+                    TransportError::MultiaddrNotSupported(addr)
+                } else {
+                    TransportError::Other(JsErr::from(err))
+                }
+            })?;
+
+        Ok(Dial {
+            inner: SendWrapper::new(promise.into()),
+        })
+    }
+}
+
+/// Future that dial a remote through an external transport.
+#[must_use = "futures do nothing unless polled"]
+pub struct Dial {
+    /// A promise that will resolve to a `ffi::Connection` on success.
+    inner: SendWrapper<wasm_bindgen_futures::JsFuture>,
+}
+
+impl fmt::Debug for Dial {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Dial").finish()
+    }
+}
+
+impl Future for Dial {
+    type Item = Connection;
+    type Error = JsErr;
+
+    fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
+        match self.inner.poll() {
+            Ok(Async::Ready(connec)) => Ok(Async::Ready(Connection::new(connec.into()))),
+            Ok(Async::NotReady) => Ok(Async::NotReady),
+            Err(err) => Err(JsErr::from(err)),
+        }
+    }
+}
+
+/// Stream that listens for incoming connections through an external transport.
+#[must_use = "futures do nothing unless polled"]
+pub struct Listen {
+    /// Iterator of `ListenEvent`s.
+    iterator: SendWrapper<js_sys::Iterator>,
+    /// Promise that will yield the next `ListenEvent`.
+    next_event: Option<SendWrapper<wasm_bindgen_futures::JsFuture>>,
+    /// List of events that we are waiting to propagate.
+    pending_events: VecDeque<ListenerEvent<FutureResult<Connection, JsErr>>>,
+}
+
+impl fmt::Debug for Listen {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Listen").finish()
+    }
+}
+
+impl Stream for Listen {
+    type Item = ListenerEvent<FutureResult<Connection, JsErr>>;
+    type Error = JsErr;
+
+    fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
+        loop {
+            if let Some(ev) = self.pending_events.pop_front() {
+                return Ok(Async::Ready(Some(ev)));
+            }
+
+            if self.next_event.is_none() {
+                let ev = self.iterator.next()?;
+                if !ev.done() {
+                    let promise: js_sys::Promise = ev.value().into();
+                    self.next_event = Some(SendWrapper::new(promise.into()));
+                }
+            }
+
+            let event = if let Some(next_event) = self.next_event.as_mut() {
+                let e = ffi::ListenEvent::from(try_ready!(next_event.poll()));
+                self.next_event = None;
+                e
+            } else {
+                return Ok(Async::Ready(None));
+            };
+
+            for addr in event
+                .new_addrs()
+                .into_iter()
+                .flat_map(|e| e.to_vec().into_iter())
+            {
+                let addr = js_value_to_addr(&addr)?;
+                self.pending_events
+                    .push_back(ListenerEvent::NewAddress(addr));
+            }
+
+            for upgrade in event
+                .new_connections()
+                .into_iter()
+                .flat_map(|e| e.to_vec().into_iter())
+            {
+                let upgrade: ffi::ConnectionEvent = upgrade.into();
+                self.pending_events.push_back(ListenerEvent::Upgrade {
+                    listen_addr: upgrade.local_addr().parse()?,
+                    remote_addr: upgrade.observed_addr().parse()?,
+                    upgrade: futures::future::ok(Connection::new(upgrade.connection())),
+                });
+            }
+
+            for addr in event
+                .expired_addrs()
+                .into_iter()
+                .flat_map(|e| e.to_vec().into_iter())
+            {
+                let addr = js_value_to_addr(&addr)?;
+                self.pending_events
+                    .push_back(ListenerEvent::AddressExpired(addr));
+            }
+        }
+    }
+}
+
+/// Active stream of data with a remote.
+pub struct Connection {
+    /// The FFI object.
+    inner: SendWrapper<ffi::Connection>,
+
+    /// The iterator that was returned by `read()`.
+    read_iterator: SendWrapper<js_sys::Iterator>,
+
+    /// Reading part of the connection.
+    read_state: ConnectionReadState,
+
+    /// When we write data using the FFI, a promise is returned containing the moment when the
+    /// underlying transport is ready to accept data again. This promise is stored here.
+    /// If this is `Some`, we must wait until the contained promise is resolved to write again.
+    previous_write_promise: Option<SendWrapper<wasm_bindgen_futures::JsFuture>>,
+}
+
+impl Connection {
+    /// Initializes a `Connection` object from the FFI connection.
+    fn new(inner: ffi::Connection) -> Self {
+        let read_iterator = inner.read();
+
+        Connection {
+            inner: SendWrapper::new(inner),
+            read_iterator: SendWrapper::new(read_iterator),
+            read_state: ConnectionReadState::PendingData(Vec::new()),
+            previous_write_promise: None,
+        }
+    }
+}
+
+/// Reading side of the connection.
+enum ConnectionReadState {
+    /// Some data have been read and are waiting to be transferred. Can be empty.
+    PendingData(Vec<u8>),
+    /// Waiting for a `Promise` containing the next data.
+    Waiting(SendWrapper<wasm_bindgen_futures::JsFuture>),
+    /// An error occurred or an earlier read yielded EOF.
+    Finished,
+}
+
+impl fmt::Debug for Connection {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Connection").finish()
+    }
+}
+
+impl io::Read for Connection {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize, io::Error> {
+        loop {
+            match mem::replace(&mut self.read_state, ConnectionReadState::Finished) {
+                ConnectionReadState::Finished => break Err(io::ErrorKind::BrokenPipe.into()),
+
+                ConnectionReadState::PendingData(ref data) if data.is_empty() => {
+                    let iter_next = self.read_iterator.next().map_err(JsErr::from)?;
+                    if iter_next.done() {
+                        self.read_state = ConnectionReadState::Finished;
+                    } else {
+                        let promise: js_sys::Promise = iter_next.value().into();
+                        let promise = SendWrapper::new(promise.into());
+                        self.read_state = ConnectionReadState::Waiting(promise);
+                    }
+                    continue;
+                }
+
+                ConnectionReadState::PendingData(mut data) => {
+                    debug_assert!(!data.is_empty());
+                    if buf.len() <= data.len() {
+                        buf.copy_from_slice(&data[..buf.len()]);
+                        self.read_state =
+                            ConnectionReadState::PendingData(data.split_off(buf.len()));
+                        break Ok(buf.len());
+                    } else {
+                        let len = data.len();
+                        buf[..len].copy_from_slice(&data);
+                        self.read_state = ConnectionReadState::PendingData(Vec::new());
+                        break Ok(len);
+                    }
+                }
+
+                ConnectionReadState::Waiting(mut promise) => {
+                    let data = match promise.poll().map_err(JsErr::from)? {
+                        Async::Ready(ref data) if data.is_null() => break Ok(0),
+                        Async::Ready(data) => data,
+                        Async::NotReady => {
+                            self.read_state = ConnectionReadState::Waiting(promise);
+                            break Err(io::ErrorKind::WouldBlock.into());
+                        }
+                    };
+
+                    // Try to directly copy the data into `buf` if it is large enough, otherwise
+                    // transition to `PendingData` and loop again.
+                    let data = js_sys::Uint8Array::new(&data);
+                    let data_len = data.length() as usize;
+                    if data_len <= buf.len() {
+                        data.copy_to(&mut buf[..data_len]);
+                        self.read_state = ConnectionReadState::PendingData(Vec::new());
+                        break Ok(data_len);
+                    } else {
+                        let mut tmp_buf = vec![0; data_len];
+                        data.copy_to(&mut tmp_buf[..]);
+                        self.read_state = ConnectionReadState::PendingData(tmp_buf);
+                        continue;
+                    }
+                }
+            }
+        }
+    }
+}
+
+impl tokio_io::AsyncRead for Connection {}
+
+impl io::Write for Connection {
+    fn write(&mut self, buf: &[u8]) -> Result<usize, io::Error> {
+        if let Some(mut promise) = self.previous_write_promise.take() {
+            match promise.poll().map_err(JsErr::from)? {
+                Async::Ready(_) => (),
+                Async::NotReady => {
+                    self.previous_write_promise = Some(promise);
+                    return Err(io::ErrorKind::WouldBlock.into());
+                }
+            }
+        }
+
+        debug_assert!(self.previous_write_promise.is_none());
+        self.previous_write_promise = Some(SendWrapper::new(
+            self.inner.write(buf).map_err(JsErr::from)?.into(),
+        ));
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> Result<(), io::Error> {
+        // There's no flushing mechanism. In the FFI we consider that writing implicitly flushes.
+        Ok(())
+    }
+}
+
+impl tokio_io::AsyncWrite for Connection {
+    fn shutdown(&mut self) -> Poll<(), io::Error> {
+        // Shutting down is considered instantaneous.
+        self.inner.shutdown().map_err(JsErr::from)?;
+        Ok(Async::Ready(()))
+    }
+}
+
+impl Drop for Connection {
+    fn drop(&mut self) {
+        self.inner.close();
+    }
+}
+
+/// Returns true if `err` is an error about an address not being supported.
+fn is_not_supported_error(err: &JsValue) -> bool {
+    if let Some(err) = err.dyn_ref::<js_sys::Error>() {
+        if String::from(err.name()) == "NotSupportedError" {
+            true
+        } else {
+            false
+        }
+    } else {
+        false
+    }
+}
+
+/// Turns a `JsValue` containing a `String` into a `Multiaddr`, if possible.
+fn js_value_to_addr(addr: &JsValue) -> Result<Multiaddr, JsErr> {
+    if let Some(addr) = addr.as_string() {
+        Ok(addr.parse()?)
+    } else {
+        Err(JsValue::from_str("Element in new_addrs is not a string").into())
+    }
+}
+
+/// Error that can be generated by the `ExtTransport`.
+pub struct JsErr(SendWrapper<JsValue>);
+
+impl From<JsValue> for JsErr {
+    fn from(val: JsValue) -> JsErr {
+        JsErr(SendWrapper::new(val))
+    }
+}
+
+impl From<libp2p_core::multiaddr::Error> for JsErr {
+    fn from(err: libp2p_core::multiaddr::Error) -> JsErr {
+        JsValue::from_str(&err.to_string()).into()
+    }
+}
+
+impl From<JsErr> for io::Error {
+    fn from(err: JsErr) -> io::Error {
+        io::Error::new(io::ErrorKind::Other, err.to_string())
+    }
+}
+
+impl fmt::Debug for JsErr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self)
+    }
+}
+
+impl fmt::Display for JsErr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Some(s) = self.0.as_string() {
+            write!(f, "{}", s)
+        } else if let Some(err) = self.0.dyn_ref::<js_sys::Error>() {
+            write!(f, "{}", String::from(err.message()))
+        } else if let Some(obj) = self.0.dyn_ref::<js_sys::Object>() {
+            write!(f, "{}", String::from(obj.to_string()))
+        } else {
+            write!(f, "{:?}", &*self.0)
+        }
+    }
+}
+
+impl error::Error for JsErr {}