Nix, a four-star prospect from Pinson, Alabama, is the son of former Auburn quarterback Pat Nix, who played for the Tigers from 1992 to 1995.
Nix, who enrolled early in January, beat out freshman Joey Gatewood for the job. Gatewood, a former four-star prospect, redshirted last season.
Coach Gus Malzahn informed the team of his decision earlier Tuesday.
Nix won back-to-back state championships at Pinson Valley High School and set state records during his prep career with more than 12,000 total yards and 161 total touchdowns.
He is the first true freshman to earn the starting job during Malzahn's six seasons at Auburn.
No. 16 Auburn opens the season against No. 11 Oregon and its star quarterback, Justin Herbert, on Aug. 31 at AT&T Stadium in Arlington, Texas.