Sierra Entertainment, Inc. formerly On-Line Systems and later Sierra On-Line, Inc. was an American video game developer and publisher based in Bellevue, Washington.

Founded in 1979 as On-Line Systems, by Ken and Roberta Williams. The company was acquired by CUC International in July 1996, and sub-organized under its CUC Software subsidiary in September 1996. CUC Software later was renamed to Cendant Software in 1997, and Cendant Software was then acquired by Havas Interactive later renamed Vivendi Universal Games and Vivendi Games in December 1998.

In 2008, Vivendi Games was acquired by and merged with Activision, and Sierra Entertainment was dissolved shortly thereafter.

The "Sierra" brand name was relaunched by Activision in 2014, and exists as a publishing label under which Sierra's legacy titles are re-released, as well as some independent titles.