ULTRIX was Digital Equipment Corporation's proprietary UNIX operating system for the VAX and MIPS-based DECstation product lines, developed during the 1980s and early 1990s. Early releases were derived from BSD UNIX, while later versions combined significant portions of both BSD and AT&T System V technologies. ULTRIX became DEC's primary TCP/IP-oriented UNIX platform during the rise of workstation computing, networking, and distributed systems, and was widely used in universities, research laboratories, engineering environments, and corporate infrastructure. It was especially notable for strong networking support, DECnet integration, and compatibility with DEC's extensive range of hardware, from MicroVAX systems to large VAX servers and graphical DECstations.

Later ULTRIX releases introduced the ULTRIX Worksystem Software (UWS) environment, providing the DECwindows graphical desktop based initially on X11 and the XUI toolkit before the industry transition to Motif. This allowed DECstations and VAXstations to function as full UNIX workstations with integrated networking, graphical applications, and remote administration capabilities. ULTRIX remained DEC's primary UNIX offering until the company shifted strategic focus toward OSF/1 on Alpha systems, which later evolved into Digital UNIX and eventually Tru64 UNIX. Despite its retirement, ULTRIX remains historically significant as one of the major commercial UNIX platforms of the workstation era and as an important bridge between BSD UNIX traditions and enterprise networking environments.