The win helped to hand the Spanish La Liga champions their first home defeat of the season.
The defeat came on a day when they were celebrating their title triumph.
With the league title in the bag, there was an end-of-season feel at Camp Nou.
The fans waited to see the post-game celebrations when FC Barcelona would lift the La Liga trophy.
However, Real Sociedad made sure it would not go all FC Barcelona’s way as they got in front quickly, with Merino scoring with a tidy finish in the fifth minute.
They extended their lead with Sorloth in the 72nd minute and Robert Lewandowski headed in a consolation for FC Barcelona from close range in the 90th minute.
With three games left, fourth-placed Real Sociedad moved on to 65 points and tightened their grip on the last Champions League qualification spot.
They are five points ahead of fifth-placed Villarreal who beat Girona 2-1 earlier on Saturday.
Champions FC Barcelona has an unassailable 14-point lead over Real
Madrid in second place, who play at Valencia on Sunday.
Atletico Madrid is third with 69 points.
(Reuters/NAN)
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