Real Sociedad damaged Barcelona's title defence ambitions as Goncalo Guedes gave them a surprise 2-1 LaLiga victory on Sunday as Hansi Flick's side fell to a first defeat in 12 matches.
The Catalan giants, who hit the woodwork four times and had two goals disallowed, now only lead rivals Real Madrid by a single point at the top of the table after Alvaro Arbeloa's side beat Levante on Saturday.
Hosts Real Sociedad, now unbeaten in four games under new American coach Pellegrino Matarazzo, had Carlos Soler sent off late on but managed to hold on to triumph and climb up to eighth.
"I think we deserved to win today, we had a lot of chances, but in the end you have to put them away," Barca midfielder Frenkie de Jong told DAZN.
"We played well but we didn't win... their goalkeeper had a very good game."
Sociedad shot-stopper Alex Remiro made several crucial saves.
"Three great points and a great game," said Remiro.
"We've started this year in great form... this is the energy we have, how we've changed (recently)."
In a frantic start to a compelling game at the rainy Reale Arena, both teams had goals ruled out.
La Real had the ball in the net inside 30 seconds when Mikel Oyarzabal headed home Guedes's cross, but had strayed offside.
At the other end, Barcelona's Fermin Lopez drilled home from range but Dani Olmo had committed a foul in the build-up and it was ruled out.
With teenage winger Lamine Yamal keen to take on defenders, Barca got in often down the right flank. The 18-year-old teed up Olmo who fired high over the bar when he might have sent the Catalans ahead.
Yamal found the net himself but the goal was ruled out for an extremely tight offside, before Real Sociedad took the lead against the run of play.
Oyarzabal lashed home Guedes's cross with a blistering volley inside Joan Garcia's near post.
Yamal was felled just inside the area before halftime but although the referee pointed to the spot, a VAR review showed that the youngster was offside again and it was cancelled out.
REMIRO HEROICS
Barcelona turned up the pressure in the second half and Real Sociedad were left depending on Remiro and the frame of the goal to keep the Catalans at bay.
Olmo crashed a shot against the post from Lopez's cross and then Remiro denied the Barca midfielder with a fine near-post save.
Remiro then tipped substitute Robert Lewandowski's header against the crossbar with the save of the night.
Another Barca substitute, Marcus Rashford, eventually pulled the champions level, heading Yamal's cross home after 70 minutes.
However, the hosts immediately regained their advantage after Barca goalkeeper Garcia initially saved Carlos Soler's shot. The midfielder was able to gather the rebound and cross for Guedes, who fired La Real ahead again.
They should have got a third when Garcia, out of his goal, had no chance of saving Oyarzabal's effort but Pau Cubarsi headed the ball off the line.
Barcelona came within centimetres of an equaliser when Joao Cancelo, on his second debut for the club, crossed for Jules Kounde, who headed against the crossbar.
Soler was dismissed for an ugly foul on Pedri before nine minutes of stoppage time were added on.
Rashford hit the post directly from a corner kick as Barca pushed until the end but could not find a way through.