Rodrigo De Paul opened the scoring for the visitors but they then had Nicolas Otamendi wrongly sent off before their former forward equalised from an offside position
Los Che looked set to claim three precious points in their bid to qualify for next season's Champions League when Rodrigo De Paul opened the scoring in the 61st minute.
However, the visitors then had Nicolas Otamendi controversially sent off before Aritz Aduriz equalised from an offside position in the dying seconds of normal time.
Valencia had arrived in Basque Country requiring a victory to both keep fifth-placed Sevilla at bay in the battle for fourth spot, and keep up the pressure on Atletico Madrid in third.
Nuno Santo's men dominated possession in a shockingly bad opening half but did absolutely nothing with it. Athletic did, at least, carry something resembling a goalscoring threat from set-pieces but they were dreadful on the ball and seemingly more concerned with breaking up the play with niggly fouls.
Thankfully, the second half brought about a welcome and sudden improvement in quality, with Xabier Etxeita's header from a corner drawing a spectacular one-handed save out of Diego Alves in the Valencia goal just seconds after the restart.
Valencia had another major escape in the 54th minute when Otamendi acrobatically cleared Aduriz's goal-bound header off the line.
However, it was Los Che who belatedly broke the deadlock just after the hour mark, with a sublime dummy from Paco Alcacer affording Rodrigo De Paul sufficient time to take a touch to steady himself before calmly stroking the ball into the bottom right corner.
Valencia then had two glorious chances to make the game safe, but Rodrigo Moreno wasted both. The Spanish attacker dragged his volley horribly wide after being brilliantly picked out at the back post by substitute Alvaro Negredo.
Rodrigo squandered an even better opening when he was put clean through on goal by a quick free-kick but he criminally shot straight at Gorka Iraizoz.
Valencia would be made to pay for their profligacy - primarily because of the incompetence of the officials.
Firsty, Otamendi was inexplicably shown a straight red card after getting to the ball first in a 50-50 challenge with Mikel San Jose in midfield.
Then, in the final minute of normal time, the offside flag incorrectly stayed down when Aduriz latched onto a deflected effort from Borja Viguera to fire home from just a yard out.
It was a case of insult added to injury for Valencia, who were not only cruelly denied two points but also lost Pablo Piatti to a hamstring injury during the first half. All things considered, it was a costly evening for Los Che in every sense.