Sven Kums' second-half penalty was enough for Gent to down Valencia in Belgium on Wednesday night and earn the hosts an historic first win in the Champions League.
Gent, who had taken one point from their opening three games, revived hopes of reaching the knockout stage of the competition thanks to the 1-0 victory.
The home side broke the deadlock four minutes into the second half when Kums converted the spot-kick after a handball from Valencia's Antonio Barragan.
Gent had been unlucky not to take a 17th-minute lead when Danijel Milicevic's flying header hit the right post.
Valencia have remained second in Group H, six points adrift of pool leaders Zenit, who won 2-0 at Lyon to seal a spot in the next round.
The outcome at the Ghelamco Arena has lifted Gent to third place, one point adrift of Valencia and three points clear of Lyon.
For a fourth straight game, Valencia coach Nuno Espirito Santo left Alvaro Negredo out of his squad with the former Manchester City striker having to watch the encounter from the stands.
Instead, Nuno opted to start Santi Mina alongside Sofiane Feghouli and Paco Alcacer in attack.
The hosts, unbeaten in Belgium's First League, came out strong and in the 17th minute, Milicevic's header from Kenny Saief's cross beat visiting goalkeeper Jaume Domenech but found the woodwork.
Shortly after, Thomas Foket fed Saief in the box and his left-footed attempt from eight yards was wide.
As Valencia struggled to break Gent's well-placed defence, the hosts looked more dangerous.
In the 35th minute, Nana Asare set up Milicevic in the heart of the area and his left-footed attempt was met with a fine save by Domenech.
The Spaniards made their first substitution on the stroke of half-time, with Pablo Piatti replacing Mina.
Valencia's struggles continued after the break and Gent were given a golden opportunity to break the deadlock in the 49th minute after Barragan's handball when the referee pointed to the penalty spot.
Domenech got a touch on Kums' spot kick but could not avoid the ball from finding the back of the net.
Valencia went forward and in the 69th minute, Shkodran Mustafi's powerful header from PIatti's cross was off target.
Shortly after, Dani Parejo drilled a long-range attempt just over the bar.
With 10 minutes remaining, Gent goalkeeper Matz Sels was called on to make his first save of the evening.
Parejo set up Alcacer in the area and his right-footed shot was kept out by Sels.
Valencia could not find the equaliser as they lost for the second time in the group.
Los Che visit Zenit on Nov. 24 while Gent next take on Lyon in France.