Coach Xavi Hernandez’s side won the title for the 27th time, four rounds from the end, after beating neighbors Espanyol 4-2 last week.
Camp Nou fans felt the season was over and everyone was waiting for the celebrations after the final whistle.
But Sociedad wanted to disturb the atmosphere and quickly advanced thanks to Mikel Merino with a perfect touch in the fifth minute.
Alexander Sorloth cemented the visitors’ superiority in the 72nd minute, while Robert Lewandowski scored an honorary goal for Barcelona with a close-range header in the 90th minute, extending his lead in the competition’s top scorer list to 22 goals. .
Three games before the end, Sociedad tightened their grip on fourth place in Champions League qualifying with 65 points, five points behind fifth-placed Villarreal, who beat Girona 2-1 earlier in the day.
Barcelona are 14 points clear of second-placed Real Madrid, who face Valencia on Sunday, and Atletico Madrid are third on 69 points.
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