Barca's captain decided the game and the title in the 61st minute as he received a header from Arturo Vidal, effortlessly freeing himself from two defenders with his right foot before using his left to plant the ball into the far bottom corner.
Cries of "champions" rang out from the 91,000 supporters crammed into the Nou Camp but Barca's players dropped their concentration and Jose Luis Morales should have levelled for the visitors but blasted over the bar.
Levante also hit the post in the final stages as Barca were left clinging on to their narrow advantage deep into stoppage-time but relief came when they cleared a corner and the final whistle sounded.
Barca lead the standings on 83 points after 35 games, an unassailable nine clear of second-placed Atletico Madrid with three matches left because they have a superior head-to-head record with Atleti this season.
Ernesto Valverde's side have now taken the first step in their bid to win a third treble in a decade.
They face Liverpool in the Champions League semi-finals and will play Valencia in the Copa del Rey final on May 25.
