RISC/os is an operating system distributed by MIPS Computer Systems, Inc. during the 1980s and 1990s for use with their MIPS Magnum line of computer workstations.

RISC/os was based largely on the 4.3 BSD UNIX release, ported to the MIPS II and III architectures.

Because of this early BSD heritage, RISC/os was limited in comparison to modern UNIX variants—for example, even the latest releases of RISC/os did not support dynamically-linked libraries (DLLs).

RISC/os is not to be confused with the similarly named RISC OS, an operating system originally developed by Acorn Computers Ltd.