The 1990–91 Primeira Divisão was the 57th edition of top flight of Portuguese football.
It started on 19 August 1990 with a match between Farense and Nacional, and ended on 26 May 1991.
The league was contested by 20 clubs with Porto as the defending champions.
Benfica qualified for the 1991–92 European Cup first round, Porto qualified for the 1991–92 European Cup Winners' Cup first round, and Sporting CP, Boavista and Salgueiros qualified for the 1991–92 UEFA Cup first round; in opposite, Tirsense, Vitória de Setúbal, Estrela da Amadora, Belenenses and Nacional were relegated to the Liga de Honra.
Portimonense and Feirense were consigned to the Liga de Honra following their final classification in 1989-90 season.