Xenix is a discontinued version of the Unix operating system, licensed by Microsoft from AT&T in the late 1970s. The Santa Cruz Operation (SCO) later acquired exclusive rights to the software, and eventually superseded it with SCO UNIX (now known as SCO OpenServer).

Xenix was Microsoft's version of Unix intended for use on microcomputers; because Microsoft was not able to license the "UNIX" name itself, they gave it an original name.

Microsoft purchased a license for Version 7 Unix from AT&T in 1979, and announced on August 25, 1980 that it would make it available for the 16-bit microcomputer market. The initial port of Xenix was to the Zilog Z8000 series and subsequently to the Intel 8086/8088 architecture ported by The Santa Cruz Operation.

Xenix varied from its 7th Edition origins by incorporating elements from BSD, and soon (for a time) possessed the most widely installed base of any Unix flavor due to the popularity of the inexpensive x86 processor.

Microsoft did not sell Xenix directly to end users; instead, they licensed it to software OEMs such as Intel, Tandy, Altos and SCO, who then ported it to their own proprietary computer architectures. Microsoft Xenix originally ran on the PDP-11; the first port was for the Zilog Z8001 16-bit processor. Altos shipped a version for their Intel 8086 based computers early in 1982, Tandy Corporation shipped TRS-XENIX for their 68000-based systems in January 1983, and SCO released their port to the IBM PC in September 1983.