LaLiga champs Barcelona suffered a defeat on the opening day of the season after a superb winner from veteran Aritz Aduriz handed Athletic Bilbao victory.
Barca produced a lacklustre performance and were punished when substitute Aduriz, 38, found the net with a stunning 89th minute scissor-kick.
Luis Suarez and Rafinha both hit the woodwork for Barca but the visitors struggled to impress despite fielding new 107m signing Antoine Griezmann.
Suarez was forced off with an injury.
Former Spain forward Aduriz has already announced he is retiring at the end of the season after a career spanning 20 years.
He threw himself into the air and acrobatically volleyed past Marc-Andre ter Stegen to earn Athletic Bilbao a first league win over Barcelona since 2013.