A third-minute strike by Ahmed Sharahili and two goals from Romarinho secured victory for Al-Ittihad, who made amends for losing the title on the final day of last season.
The Jeddah club have 69 points, five more than Al-Nassr who drew 1-1 with Al-Ettifaq, with one round left to play. Saturday's result sealed Al-Ittihad their ninth league title.
Sharahili gave the visitors the perfect start, turning in Igor Coronado's free kick at the far post in the third minute.
Romarinho headed in the second from another Coronado free kick during the first-half stoppage time. The Brazilian scored his second five minutes from the end.
The result handed former Tottenham Hotspur and Wolverhampton Wanderers coach Nuno Espirito Santo his first top-flight league title.
In Dammam, Ronaldo endured another frustrating outing and was substituted in the second half as Al-Nassr were held to a draw by Al-Ettifaq.
Youssoufou Niakate put Al-Ettifaq ahead two minutes before the break, while Brazilian Luiz Gustavo equalised for Al-Nassr in the 56th minute.
Ronaldo will finish his first season in the country empty-handed after joining the club in January.
