System 7 (codenamed Big Bang and sometimes retroactively called Mac OS 7) is a version of the Macintosh operating system, single-user operating system of the Macintosh computer, introduced in May 13, 1991. It succeeded System 6, and was the main Mac operating system until it was succeeded by Mac OS 8 in 1997. Features added with the System 7 release included cooperative multitasking, virtual memory, personal file sharing, an improved user interface look, QuickTime, and QuickDraw 3D.

"System 7" is often used generically to refer to all 7.x versions. With the release of version 7.6 in 1997, Apple officially renamed the operating system "Mac OS", a name which had first appeared on System 7.5.1's boot screen. System 7 was developed for the Motorola 68k processor, but was ported to the PowerPC after Apple adopted the new processor.