IRIX is a flavour of the UNIX System V computer operating system developed by Silicon Graphics, Inc. to run natively on their MIPS architecture workstations and servers. It was based on UNIX System V with BSD extensions. IRIX was the first operating system to include the XFS file system.

The last major version of IRIX was IRIX 6.5 which was released in May 1998. New minor versions of IRIX 6.5 were released every quarter until 2005; since then there have been four further minor releases. Through version 6.5.22, there were two branches of each release: a maintenance release (identified by an m suffix to the version number) that included only fixes to the original IRIX 6.5 code, and a feature release (with an f suffix) that included improvements and enhancements. An overlay upgrade from 6.5.x to the 6.5.22 maintenance release is available as a free download, whereas versions 6.5.23 and higher require an active Silicon Graphics support contract, despite only running on Silicon Graphics hardware.