System 7 (codenamed "Big Bang" and sometimes retrospectively called Mac OS 7) is a single-user graphical user interface-based operating system for Macintosh computers and was part of the classic Mac OS line of operating systems. It was introduced on May 13, 1991, by Apple Computer. It succeeded System 6, and was the main Macintosh operating system until it was succeeded by Mac OS 8 in 1997. Features added with the System 7 release included virtual memory, personal file sharing, QuickTime, QuickDraw 3D, and an improved user interface.

"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 Macs that used the Motorola 680x0 line of processors, but was ported to the PowerPC after Apple adopted the new processor.

System 7 was the first Apple operating system to be available on CD, although it shipped on a set of 15 floppy disks initially. Unlike earlier systems, System 7 itself did not come bundled with major software packages. Newly purchased Macintosh computers had System 7 installed and were often bundled with software such as HyperCard, At Ease and Mouse Practice. Later, the Macintosh Performa series added various software bundles including third-party software such as ClarisWorks, The New Grolier Multimedia Encyclopedia, Microsoft Bookshelf, Spectre VR and Power Pete. Since System 7 was introduced before the Internet came to popular attention, software such as MacTCP, FreePPP and Netscape were not included at first, but was later available on disk from Internet service providers and bundled with books such as Adam C. Engst's Internet Starter Kit for Macintosh. PowerPC Macintoshes included Graphing Calculator. System 7 also included AppleTalk networking and file sharing software in the form of system extensions and control panels.

The basic utilities installed by default with System 7 included TeachText (superseded by the more flexible SimpleText in later versions) for basic text editing tasks and reading readme documents. Also available on the additional "Disk Tools" floppy disk are Disk First Aid for disk repair and Apple HD SC Setup for initializing and partitioning disks.