Barcelona suffered defeat in their first match since winning La Liga as Real Sociedad boosted their Champions League qualification hopes.
Sociedad scored in the fifth minute when Alexander Sorloth set up Mikel Merino to finish from close range.
Sorloth then added a second in the 72nd minute to give the visitors a deserved two-goal lead.
Robert Lewandowski pulled a goal back in injury time but the champions were unable to find a leveller.
Barcelona sealed their first La Liga title since 2018-19 with four games to spare last weekend by winning away to city rivals Espanyol.
On Saturday, they lifted the La Liga trophy in front of their home supporters after the match with Sociedad at the Nou Camp.
With the win, fourth-placed Sociedad restored their five-point gap over Villarreal in fifth.
Line-ups
Barcelona
Formation 4-3-3
- 1ter Stegen
- 28Balde
- 23KoundéSubstituted forAlonsoat 45′minutesBooked at 79mins
- 15Christensen
- 18Alba
- 19KessiéSubstituted forF Torresat 63′minutes
- 5BusquetsBooked at 72minsSubstituted forGarcíaat 84′minutes
- 21F de JongBooked at 85mins
- 22RaphinhaSubstituted forAnsu Fatiat 63′minutes
- 9LewandowskiBooked at 53mins
- 7DembéléSubstituted forTorreat 86′minutes
Substitutes
- 10Ansu Fati
- 11F Torres
- 13Peña Sotorres
- 17Alonso
- 20Roberto
- 24García
- 31Astralaga
- 32Torre
Real Sociedad
Formation 4-2-3-1
- 1Remiro Gargallo
- 6Elustondo
- 5ZubeldiaBooked at 8minsSubstituted forPachecoat 81′minutes
- 24Le Normand
- 15Rico
- 4IllarramendiBooked at 37minsSubstituted forGorosabelat 58′minutes
- 3Zubimendi
- 11ChoSubstituted forKuboat 58′minutesBooked at 79mins
- 8Merino
- 7BarrenetxeaSubstituted forMarínat 73′minutes
- 19SørlothSubstituted forFernándezat 81′minutesBooked at 85mins
Substitutes
- 2Sola
- 9Fernández
- 10Oyarzabal
- 12Muñoz
- 13Zubiaurre
- 14Kubo
- 16Guevara
- 17Navarro
- 18Gorosabel
- 20Pacheco
- 22Turrientes
- 42Marín
- Referee:
- Javier Alberola Rojas
- Attendance:
- 88,049
Live Text
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Match ends, Barcelona 1, Real Sociedad 2.
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Second Half ends, Barcelona 1, Real Sociedad 2.
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Foul by Robert Lewandowski (Barcelona).
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Mikel Merino (Real Sociedad) wins a free kick in the defensive half.
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Offside, Real Sociedad. Mikel Merino tries a through ball, but Carlos Fernández is caught offside.
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Attempt blocked. Robert Lewandowski (Barcelona) right footed shot from outside the box is blocked. Assisted by Ansu Fati.
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Goal! Barcelona 1, Real Sociedad 2. Robert Lewandowski (Barcelona) header from the centre of the box to the high centre of the goal. Assisted by Ferran Torres with a cross.
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Pablo Torre (Barcelona) wins a free kick in the attacking half.
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Foul by Mikel Merino (Real Sociedad).
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Attempt missed. Pablo Marín (Real Sociedad) left footed shot from the left side of the box is high and wide to the left. Assisted by Mikel Merino.
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Substitution, Barcelona. Pablo Torre replaces Ousmane Dembélé.
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Frenkie de Jong (Barcelona) is shown the yellow card.
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Carlos Fernández (Real Sociedad) is shown the yellow card for a bad foul.
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Frenkie de Jong (Barcelona) wins a free kick on the right wing.
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Foul by Carlos Fernández (Real Sociedad).
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Substitution, Barcelona. Eric García replaces Sergio Busquets.
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Attempt saved. Diego Rico (Real Sociedad) left footed shot from outside the box is saved in the centre of the goal.
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Attempt blocked. Takefusa Kubo (Real Sociedad) left footed shot from the centre of the box is blocked. Assisted by Mikel Merino.
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Substitution, Real Sociedad. Carlos Fernández replaces Alexander Sørloth.
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Substitution, Real Sociedad. Jon Pacheco replaces Igor Zubeldia.
