Bilbao said the new deal meant Aduriz could only leave if another club offers 40 million euros ($40 million) for the veteran striker.
Aduriz has scored six goals in the Spanish championship and another six in the Europa League -- including five in one game against Genk -- and is joint leader in the tournament.
His current deal ends on June 30 next year, but Bilbao said the contract now runs until June 30, 2018.