The only goal of the game came on 79 minutes as Sergio Canales and Cristian Tello combined in a swift counterattack to set up Juanmi to coolly slot an effort past Barca goalkeeper Marc-Andre ter Stegen.
Prior to the match Pedri was handed his Kopa Trophy and Alexia Putellas he Ballon d'Or Femenin award.
Alex Moreno went close to scoring for Betis just before half-time after evading Sergino Dest, but fired just wide.
Barca upped the pressure in the second half as Ousmane Dembele, who replaced Coutinho on 59 minutes, twice went close.
But Betis ensured they took the three points through Juanmi's late goal to hand Xavi his first defeat as Barca boss.