Both sides had opportunities in the first period, but neither could make the breakthrough with the scores level at the interval.
Benat had the opportunity to send Bilbao ahead from the penalty spot in the early stages of the half, but he could not register against his former club.
Jorge Molina made no mistake from inside the box just past the hour mark, however, with his effort sending Betis into the lead.
Read how it all unfolded in Sports Mole's minute-by-minute live commentary of the action below.
BILBAO: Iraizoz; Iraola, San Jose, Laporte, Gurpegi; Rico, Benat, Herrera; Susaeta, Aduriz, Ibai
REAL BETIS: Sara; Chica, Amaya, Paulao, Nacho; Matilla, Torres, Juanfran, Chuli, Salva; Molina