Ultrix was a DEC version of Unix. The initial development of Unix occurred on DEC equipment, notably their PDP-7 and 11 (Programmable Data Processor). Later DEC computers, such as their VAX systems were also popular platforms to run Unix on, the first port to VAX was in 1978 (the VAX was only released in October 1977). However DEC pushed their own proprietary VMS operating system for a long time before they acknowledged Unix.

The first version of Ultrix was released in 1982. It was an attempt to bridge the void between Unix and VMS, providing support for DECnet and other proprietary DEC equipment. It ran on PDP-11 minicomputers, DECstation workstations, which used MIPS RISC processors, and on DEC VAX-machines.

Ultrix was based on BSD 4.2, with some features from 4.3 and System V Release 2. It supported TCP/IP and DECnet networking and DEC's X window system desktop environment, DECwindows (before that was based on CDE).

The last release of Ultrix in 1995 was 4.5 supporting DECstations and VAX. There have been Y2k fixes later too.