NeXTSTEP is a discontinued object-oriented, multitasking operating system based on the Mach kernel and the UNIX-derived BSD. It was developed by NeXT Computer in the late 1980s and early 1990s and was initially used for its range of proprietary workstation computers such as the NeXTcube. It was later ported to several other computer architectures.

Although relatively unsuccessful at the time, it attracted interest from computer scientists and researchers. It hosted the original development of the Electronic AppWrapper, the first commercial electronic software distribution catalog to collectively manage encryption and provide digital rights for application software and digital media, a forerunner of the modern "app store" concept. It is the platform on which Tim Berners-Lee created the first web browser, and on which id Software developed the video games Doom and Quake.

In 1996, Apple Computer acquired NeXT. Apple, who needed to release a successor to the classic Mac OS, merged NeXTSTEP and OpenStep with Apple's user environment to create Mac OS X, later renamed macOS. All of Apple's subsequent platforms, including iOS, used this OS as a base.