Sierra Entertainment Inc. (formerly Sierra On-Line) was an American video-game developer and publisher founded in 1979 as On-Line Systems by Ken and Roberta Williams. Based in Oakhurst, California, the company was last owned by Activision, a subsidiary of Activision Blizzard.

Sierra is best known today for its multiple lines of seminal graphic adventure games started in the 1980s, many of which proved influential in the history of video games. The Sierra label was absorbed by its parent company. Some franchises (such as Crash Bandicoot and Spyro the Dragon) that were published by Sierra will be published by Activision, which also announced in 2008 that it may sell the Sierra brand.

Sierra Entertainment was founded in 1979 as On-Line Systems in Simi Valley, California, by Ken and Roberta Williams. Ken Williams, a programmer for IBM, bought an Apple II microcomputer with which he planned to use to develop a FORTRAN compiler for the Apple II. At the time, his wife Roberta Williams was playing text adventure games on the Apple II. Dissatisfied with the text-only format, she realized that the graphics display capability of the Apple II could enhance the adventure gaming experience. After initial success, On-Line Systems was renamed Sierra On-Line in 1982, and the company moved to Oakhurst, California.