AT&T UNIX System V was one of the versions of the UNIX operating system. It was developed by AT&T and first released in 1989. 4 major versions of System V were released, termed Releases 1, 2, 3 and 4. System V Release 4, or SVR4, was the most successful version, and the source of several common Unix features, such as "SysV init scripts" (/etc/init.d), used to control system startup and shutdown, and the System V Interface Definition (SVID), a standard defining how System V systems should work.

While AT&T sold their own hardware which ran System V, many (perhaps most) customers ran a version from a reseller, based on AT&T's reference implementation. Popular SysV derivatives include Dell SVR4 and Bull SVR4. The most widely used versions of System V today are SCO OpenServer, based on System V Release 3, and Sun Microsystems Solaris Operating Environment and SCO UnixWare, both based on System V Release 4.

System V was an enhancement over AT&T's first commercial UNIX called System III. Traditionally, System V has been considered one of the two major "flavors" of UNIX, the other being BSD. However, with the advent of UNIX implementations developed from neither code base, such as Linux and QNX, this generalisation is not as accurate as it once was, and in any case standardisation efforts such as POSIX are tending to reduce the differences between implementations.