An end-to-end game saw Real Sociedad take an early lead on the day Barcelona were crowned kings of La Liga. After going two goals down, Barca got a consolation goal back to lighten the mood before their trophy presentation.
Mikel Merino and Alexander Sorloth struck for the visitors, who restored their five-point gap on Villarreal, fifth, in the fight for Champions League qualification.
Robert Lewandowski pulled one back late on for Barcelona, who clinched the title last weekend with a thumping 4-2 win at Catalan rivals Espanyol, and were given a guard of honour before the game. However Imanol Alguacil's side fought hard to dampen the celebrations in Catalonia with an impressive victory.
Xavi picked a strong Barcelona line-up despite already wrapping up the title, but they quickly fell behind. Jules Kounde carelessly lost the ball in a dangerous area and Sorloth fed Merino, who fired through Marc-Andre ter Stegen's legs after five minutes.
It was the first goal the German goalkeeper has conceded from open play at Camp Nou in La Liga this season, with the exception of a Ronald Araujo own goal. Ter Stegen has 25 clean sheets this season and needs one more to match the league record of 26, although it will not be easy if Barcelona defend as casually in their final three matches.
Ousmane Dembele, bright on his first start after injury, came close at the other end with a header which Alex Remiro did well to keep out. Franck Kessie had a shot deflected narrowly wide as Barcelona turned the screw in search of an equaliser.
However Barcelona's pressure waned in the second half and La Real doubled their lead when Martin Zubimendi tackled Frenkie de Jong to spark a counter-attack.
Takefusa Kubo teed up Sorloth, who fired past Ter Stegen, who has now conceded 15 La Liga goals, three fewer than the record lowest tally of 18 for a 38-match season.
Barcelona captain Sergio Busquets was given a standing ovation as he was substituted in the final stages, in his penultimate home game before he leaves in the summer.
La Liga's top scorer Lewandowski nodded home his 22nd goal of the campaign to give Real Sociedad a nervy finale but they held on for an important win. After Barcelona's joy was cut short by Espanyol fans' pitch invasion last week, they got the party started again at Camp Nou, albeit initially subdued after their defeat.