diff --git a/actioncable/CHANGELOG.md b/actioncable/CHANGELOG.md
index 9c39bbdc4e..438dfd89da 100644
--- a/actioncable/CHANGELOG.md
+++ b/actioncable/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Protect against concurrent writes to a websocket connection from
+    multiple threads; the underlying OS write is not always threadsafe.
+
+    *Tinco Andringa*
+
+
 ## Rails 5.0.0 (June 30, 2016) ##
 
 *   Fix development reloading support: new cable connections are now correctly
diff --git a/actioncable/lib/action_cable/connection/stream.rb b/actioncable/lib/action_cable/connection/stream.rb
index c250cf92fc..5695623859 100644
--- a/actioncable/lib/action_cable/connection/stream.rb
+++ b/actioncable/lib/action_cable/connection/stream.rb
@@ -1,3 +1,5 @@
+require 'thread'
+
 module ActionCable
   module Connection
     #--
@@ -11,6 +13,7 @@ module ActionCable
         @stream_send   = socket.env['stream.send']
 
         @rack_hijack_io = nil
+        @write_lock = Mutex.new
       end
 
       def each(&callback)
@@ -27,8 +30,10 @@ module ActionCable
       end
 
       def write(data)
-        return @rack_hijack_io.write(data) if @rack_hijack_io
-        return @stream_send.call(data) if @stream_send
+        @write_lock.synchronize do
+          return @rack_hijack_io.write(data) if @rack_hijack_io
+          return @stream_send.call(data) if @stream_send
+        end
       rescue EOFError, Errno::ECONNRESET
         @socket_object.client_gone
       end