DEC OSF/1 was Digital Equipment Corporation's implementation of the OSF/1 operating system developed by the Open Software Foundation during the UNIX wars of the late 1980s and early 1990s. Initially released for DEC Alpha systems, DEC OSF/1 combined the Mach microkernel architecture with large portions of BSD UNIX, while also incorporating features from System V and other UNIX traditions. The operating system was designed to replace ULTRIX as DEC's strategic UNIX platform and emphasized high performance, scalability, advanced virtual memory management, and support for 64-bit computing. DEC OSF/1 later evolved into Digital UNIX and eventually Tru64 UNIX.

DEC OSF/1 became particularly well regarded for its performance on Alpha hardware, especially in scientific computing, enterprise workloads, and large SMP systems. It introduced advanced features such as the AdvFS filesystem, sophisticated clustering support, and strong compatibility layers for existing ULTRIX applications. Combined with the exceptional performance of Alpha processors, DEC OSF/1 helped establish DEC as a major UNIX vendor during the 1990s. Although eventually discontinued following Compaq and HP’s acquisitions of DEC, the operating system remains historically significant as one of the most technically advanced commercial UNIX platforms of its era.