Unix System Laboratories (USL), sometimes written UNIX System Laboratories to follow relevant trademark guidelines of the time, was an American software laboratory and product development company that existed from 1989 through 1993. At first wholly, and then majority, owned by AT&T, it was responsible for the development and maintenance of one of the main branches of the Unix operating system, the UNIX System V Release 4 source code product. Through Univel, a partnership with Novell, it was also responsible for the development and production of the UnixWare packaged operating system for Intel architecture. In addition it developed Tuxedo, a transaction processing monitor, and was responsible for certain products related to the C++ programming language. USL was based in Summit, New Jersey, and its CEOs were Larry Dooling followed by Roel Pieper.

Created from earlier AT&T entities, USL was, as industry writer Christopher Negus has observed, the culmination of AT&T's long involvement in Unix, "a jewel that couldn't quite find a home or a way to make a profit." USL was sold to Novell in 1993.

AT&T announced the creation of the UNIX Software Operation (USO) – a separate and distinct AT&T business unit responsible for the development, marketing, and licensing of UNIX System V software – in January 1989. This was done, as a subsequent press release stated, "in order to separate AT&T's UNIX System source code business from its computer systems business," the latter a reference to AT&T Computer Systems. USO included the AT&T Data Systems Group organizations responsible for UNIX product planning and management, licensing, and marketing. Peter J. Weinberger was named chief scientist of USO while also retaining his job in the computing science research center at Bell Labs; no other Bell Labs assets were transferred to USO. The head of USO was Larry Dooling, who had been a vice-president in sales and marketing in the AT&T Data Systems Group.

Unlike the original Unix work, which had been done in the Bell Labs facility in Murray Hill, USO and the commercialization work was done a few miles away in Summit, New Jersey. This AT&T Bell Labs location was known as SF for Summit Facility.

UNIX System Laboratories, Inc., came into being as a separate subsidiary of AT&T in November 1989 and was assigned all U.S.-based AT&T Unix and USO assets. However USO continued to operate as USO until June 1990, when the reincorporation of AT&T's European and Asian Unix business operations as wholly owned subsidiaries of USL was completed. At that point the UNIX Software Operation was publicly rebranded as UNIX System Laboratories. Again, a point of emphasis was to separate the Unix-based business from AT&T's hardware-based business. The subsidiaries were known as UNIX System Laboratories Europe, Ltd., sited near Ealing Broadway in London, and UNIX System Laboratories Pacific, Ltd., located in Shiba, Tokyo. Dooling was named the initial president of USL, continuing from his position at USO.

These organizational changes were taking place in the context of the open systems movement and the ongoing Unix wars. In consequence, the pro-AT&T side Unix International (as opposed to the anti-AT&T side Open Software Foundation) declared that "In the last 18 months AT&T has made good on its commitment to treat UNIX System as the industry asset it is and to open the UNIX System V development process to the entire industry."

One unit within USL, called the UNIX System V Software business unit and headed by Michael J. DeFazio, was responsible for the development of the UNIX System V base technology. DeFazio had held a similar role within USO. The USO/USL staff was heavily involved in the creation of UNIX System V Release 4, which shipped in 1989 and was a joint project with Sun Microsystems. This work incorporated technology from a variety of Unix-based efforts, including UNIX System V, BSD, and Xenix. There were additions and new innovations as well from both the AT&T and Sun sides. System V Release 4 debuted at the Unix Expo trade show in New York in November 1989, in the form of source code availability for it as well as demonstrations from Unix International of SVR4-based applications running on seventeen different vendor platforms. End-user versions of Release 4 became available during 1990.

Next USL engaged in an especially arduous effort into trying to satisfy the requirements of the National Computer Security Center's Trusted Computer System Evaluation Criteria ("Orange Book") to the B2 level. This manifested itself in System V Release 4.1 ES (Enhanced Security), which also included generally useful features such as support for dynamic loading of kernel modules.