Barcelona suffered only their second La Liga defeat this season, losing to Real Sociedad in a match where their teenage star Lamine Yamal was sidelined due to injury.
Despite the loss, Barca remains six points clear at the top of the table, with reigning champions Real Madrid in second place and a game in hand.
The decisive goal came from Sheraldo Becker in the first half, as the hosts secured a well-deserved victory, creating several opportunities to extend their lead.
Robert Lewandowski had a first-half goal controversially disallowed for offside, contributing to a challenging evening for Hansi Flick’s side.
Historically successful at San Sebastian, the Catalans struggled to find their rhythm, appearing disorganized in both attack and defense.
The introduction of Dani Olmo for the ineffective Frenkie de Jong failed to shift the momentum in Barcelona’s favor.
With Yamal out due to a knock sustained in their Champions League win over Red Star Belgrade, Barcelona ended the match without registering a single shot on target.