Sierra Entertainment, Inc. (formerly On-Line Systems and Sierra On-Line, Inc.) was an American video game developer and publisher founded in 1979 by Ken and Roberta Williams. The company is known for pioneering the graphic adventure game genre, including the first such game, Mystery House. It is known for its graphical adventure game series King's Quest, Space Quest, Police Quest, Gabriel Knight, Leisure Suit Larry, and Quest for Glory, and as the original publisher of Valve's Half-Life series.

After seventeen years as an independent company, Sierra was acquired by CUC International in February 1996 to become part of CUC Software. However, CUC International was caught in an accounting scandal in 1998, and many of the original founders of Sierra including the Williamses left the company. Sierra remained as part of CUC Software as it was sold and renamed several times over the next few years. Sierra was formally disestablished as a company and reformed as a division of this group in August 2004. The former CUC Software group was acquired by Vivendi and branded as Vivendi Games in 2006. The Sierra division continued to operate through Vivendi Games's merger with Activision to form Activision Blizzard on July 10, 2008, but was shut down later that year. The Sierra brand was revived by Activision in 2014 to re-release former Sierra games and some independently developed games.