Jonathan J. "Jack" O'Neill is a fictional character in the MGM's military science fiction franchise Stargate, primarily as one of the main characters of the television series Stargate SG-1. Richard Dean Anderson played O'Neill in all the Stargate media since 1997, when he took over the role from actor Kurt Russell, who portrayed the character in the original Stargate film in 1994. O'Neill and Daniel Jackson are the only two characters to appear in both the original film and all three live-action Stargate television series.

Stargate is a military science fiction media franchise based on the film directed by Roland Emmerich, which he co-wrote with producer Dean Devlin. The franchise is based on the idea of an alien wormhole device that enables nearly instantaneous travel across the cosmos. The franchise began with the film Stargate, released on October 28, 1994, by Metro-Goldwyn-Mayer and Carolco, which grossed US$197 million worldwide. In 1997, Brad Wright and Jonathan Glassner created a television series titled Stargate SG-1 as a sequel to the film. This show was joined by Stargate Atlantis in 2004, Stargate Universe in 2009, and a prequel web series, Stargate Origins, in 2018. Also consistent with the same story are a variety of books, video games and comic books, as well as the direct-to-DVD movies Stargate: Children of the Gods, Stargate: The Ark of Truth, and Stargate: Continuum, which concluded the first television show after 10 seasons.