OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as the preferred operating system for IBM's "Personal System/2 (PS/2)" line of second-generation Personal Computers. OS/2 is no longer marketed by IBM, and IBM standard support for OS/2 was discontinued on December 31, 2006. Currently, Serenity Systems sells OS/2 under the brand name eComStation.

OS/2 was intended as a protected mode successor of PC-DOS and Microsoft Windows. Notably, basic system calls were modeled after MS-DOS calls; their names even started with "Dos" and it was possible to create "Family Mode" applications: text mode applications that could work on both systems. Because of this heritage, OS/2 is like Windows in many ways, but it also shares similarities with Unix and Xenix.

OS/2 is also remembered for being the first major operating system to have its own advocacy group. Team OS/2 was a grassroots, ad-hoc organization of volunteers, who promoted and supported the operating system and applications designed for it.