Quique Setien suffered his first defeat as Barcelona manager as Valencia claimed their first home win over the Spanish champions since 2007.
The hosts broke the deadlock when Jordi Alba deflected Maxi Gomez’s powerful strike past Marc-Andre ter Stegen.
Gomez, who had a first-half penalty saved by Ter Stegen, completed the scoring in the second half.
They were unsavoury scenes before the game as rival fans clashed outside the Mestalla Stadium.
Setien had enjoyed a good start to his Barca reign, guiding the club to victories over Granada in La Liga and UD Ibiza in the Copa del Rey round of 32.
The visitors were quick to gain a stranglehold on possession but were almost non-existent as an attacking force in the first half, with Valencia enjoying the better of the early chances.
Los Che were gifted a golden opportunity to take the lead when Gerard Pique felled Gomez inside the penalty area, but the Uruguayan’s spot-kick was superbly palmed to safety by Ter Stegen.