The W Window System, or simply W, is a discontinued windowing system and precursor, in name and concept, to the modern X Window System.

W was originally developed at Stanford University by Paul Asente and Brian Reid for the V operating system. In 1983, Paul Asente and Chris Kent ported the system to UNIX on the VS100, giving a copy to those working at MIT's Laboratory for Computer Science.

In 1984, Bob Scheifler of MIT replaced the synchronous protocol of W with an asynchronous alternative and named the result X.

Since this time, the X window system has gone through many fundamental changes and no longer bears any significant resemblance to W.