A/UX is Apple Computer's implementation of the Unix operating system for some of their Macintosh computers, the latest versions running on the Mac II, Quadra and Centris series of machines. A/UX was first released in 1988, with the final version (3.1.1) released in 1995. A/UX requires a 68k Mac with an FPU and a paged memory management unit (PMMU).

The operating system is based on System V.2.2 with features of V.3 and V.4 and BSD 4.2 and 4.3. It is POSIX and System V Interface Definition (SVID) compliant.

A/UX 3.x provided a graphical user interface with the familiar Finder windows, menus, and controls. The A/UX Finder is not the same program as the System 7 Finder, but a customized version adapted to run as a Unix process and designed to interact with the Unix kernel and file systems. A/UX 3.x also included a CommandShell terminal program, which offered a command line interface to the underlying Unix system, a feature which was never available on Macintosh computers running the classic Mac OS Finder. An X Window server application (called MacX) with a terminal program could also be used to interface with the system and run X Window applications directly in the Finder. Alternatively, the user could choose to run a full X11R4 session without the Finder.

By including a compatibility layer, A/UX could run Macintosh System 7.0.1 applications, Unix applications, and "hybrid" applications. A hybrid application could be a Macintosh application which called Unix system functions, or a Unix application which called Macintosh Toolbox (e.g. QuickDraw) functions. The compatibility layer used some existing Toolbox functions in the computer's ROM, while other function calls were translated into native Unix system calls.

Unfortunately for Apple and for A/UX users, the Unix niche was a costly business in the early 1990s. Apple never ported A/UX to PowerPC Macintoshes, and the company all but abandoned it by 1996. As Steve Jobs returned to Apple later on, the future of Mac OS would end up truer to NeXTSTEP than to the old Mac or traditional Unix.

A/UX users had one central source for most A/UX applications, a server at NASA called "Jagubox." Although Jagubox is down, some mirrors are still maintained.

Aside from a few isolated servers, this OS is essentially extinct.