The Catalans fell to a second successive defeat following their Clasico loss to Real Madrid at the Anoeta Stadium, with Mikel Oyarzabal's headed goal proving decisive.
Barcelona are just three points clear of Atletico Madrid and four ahead of Real Madrid in La Liga as their miserable form at Anoeta Stadium continued with a 1-0 defeat at Real Sociedad on Saturday, a result that blew the title race wide open.
Having been beaten by Madrid in the Clasico last weekend and seen both their arch rivals and fellow title challengers Atletico win earlier in the day, Barca headed to Basque country needing a positive result.
But instead they slipped to a third game without a win as the Catalan club's wait for a first away win over Sociedad since 2007 continued.
Sociedad, who came into the game on the back of a surprise 2-1 win over Sevilla, netted what proved to be the decisive goal in the fifth minute through Mikel Oyarzabal's fine header.
Barca dominated from that point on but, missing the suspended Luis Suarez, could not find a way past the in-form Geronimo Rulli, who pulled off a string of fine saves to frustrate Luis Enrique's men.
Lionel Messi had the best chance to level matters but he was left waiting for his 500th Barca goal as Sociedad held on to secure a repeat of last season's 1-0 win and blow the title race wide open.
Neymar was denied by Sociedad goalkeeper Rulli after just two minutes and Barca were quickly made to rue that missed opportunity as the hosts opened the scoring three minutes later.
Xabi Prieto's excellent delivery from the right was met by Oyarzabal with a superb looping header that left Claudio Bravo with no chance.
Barca eventually recovered their composure and began to take control of the game without creating too many opportunities, although Arda Turan was unfortunate to see his deflected close-range strike kept out by Rulli.
Sociedad started the second half in the same bright manner as they had begun the first but were grateful to Rulli for preserving the single-goal lead as he thwarted substitute Andres Iniesta with a stunning save.
Rulli dived brilliantly to his right to turn a fierce Iniesta drive from outside the area behind with a one-handed save.
Neymar then curled wide from a free-kick as Barca laboured to break down a stubborn Sociedad defence and their frustration will only have grown after Rulli produced more heroics to stop Messi from prodding in a leveller following good work from Gerard Pique.
Messi saw a late header easily saved by Rulli, who then denied Iniesta once more as Sociedad stood firm to condemn Barca to successive league defeats for the first time since November 2014.
