The R4000 is a microprocessor developed by MIPS Computer Systems that implements the MIPS III instruction set architecture (ISA). Officially announced on 1 October 1991, it was one of the first 64-bit microprocessors and the first MIPS III implementation. In the early 1990s, when RISC microprocessors were expected to replace CISC microprocessors such as the Intel i486, the R4000 was selected to be the microprocessor of the Advanced Computing Environment (ACE), an industry standard that intended to define a common RISC platform. ACE ultimately failed for a number of reasons, but the R4000 found success in the workstation and server markets.

MIPS Tech LLC, formerly MIPS Computer Systems, Inc. and MIPS Technologies, Inc., is an American fabless semiconductor design company that is most widely known for developing the MIPS architecture and a series of RISC CPU chips based on it. MIPS provides processor architectures and cores for digital home, networking, embedded, Internet of things and mobile applications.