OS/390 is an IBM operating system for the System/370 and System/390 IBM mainframe computers. It is basically a rebranded version of MVS with added Unix services. The OS/390 base includes a Communication Server that includes VTAM, the VTAM AnyNet feature, TCP/IP and TIOC. It provides SNA (3270), APPC, High Performance Routing, ATM support, sockets and RPC.

OS/390 was introduced in late 1995 and was itself rebranded in December 2001 and extended to 64 bit processors and is currently called Z/OS. IBM has announced that support for OS/390-branded versions will be discontinued in 2004.

This article (or an earlier version of it) contains material from FOLDOC's article on OS/390, used with permission.