The Costa Rica international, on loan from Arsenal, cut in off the right flank to finish with style in the 77th minute after struggling Deportivo had battled back to level at 2-2 in the second half.
Betis have 25 points from 14 games, four behind Real Madrid in third after a goal by Cristiano Ronaldo helped the champions to a 2-0 home win over Atletico Madrid in the city derby on Saturday.
Barcelona, who set a new La Liga record start for a season with their Lionel Messi-inspired 5-1 victory over Athletic Bilbao on Saturday, are top with 40 points, ahead of second-placed Atletico on 34.
Betis striker Ruben Castro headed the visitors in front after a well-worked move before the break at the Riazor and also pounced for his eighth goal of the season in the 54th with a clever dinked finish just inside the far post.
Depor responded immediately when Riki broke the offside trap to head in two minutes later and by the 60th the hosts had drawn level as the same player made the most of a poor clearance to score his second.
Defeat left promoted Depor second from bottom in the table with 11 points, one ahead of Espanyol who could only manage a 0-0 draw at Granada on new coach Javier Aguirre's debut.