Samantha "Sam" Carter (seasons 1–10 main) – A brilliant young astrophysicist and United States Air Force Captain who joins SG-1 under the command of Col. O'Neill in the pilot episode. Following her promotion to Major in season 3, she is promoted to Lieutenant Colonel early in season 8 and assumes command of SG-1.

The series repeatedly alludes to romantic feelings between Carter and O'Neill, but the relationship is never shown as consummated outside of alternate reality scenarios; however Season 9 Episode 7 alludes to their forming a relationship. Carter assists Lt. Col. Cameron Mitchell in seasons 9 and 10.

After her appearance in Stargate: The Ark of Truth, she is promoted to a full-bird Colonel and becomes the new commander of the Atlantis expedition in season 4 of Stargate Atlantis before joining SG-1 again for Stargate: Continuum. She is placed in temporary command of the SGC during the attack by a Wraith ship in the series finale of Stargate Atlantis.

Later, she is given command of the Daedalus class battlecruiser George Hammond and appears in the first episode of Stargate: Universe. Carter appears in all seasons of Stargate Atlantis and will also appear in the first Atlantis direct-to-DVD film and in the third SG-1 film.