The Costa Rica forward cut in off the right flank to finish in style in the 77th minute after struggling Deportivo had battled back to level at 2-2 in the second half.
"I think he is going to be a good player for us," Betis coach Pepe Mel said of Campbell.
"It isn't good for the lad to be spending a lot of time crossing the Atlantic," Mel added, in reference to the 20-year-old's call ups for international duty.
"In the time he has had with us, he has matured and picked up the team's way of playing but it has taken time. His development will be good for Betis, for Costa Rica and ultimately everyone."
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.
AGUIRRE DEBUT
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.
"Anything you can get at this stage is good," the former Mexico boss told a news conference after picking up their first point in four outings.
Levante were lucky to grab a late equaliser through Roger to draw 1-1 at Celta Vigo after the hosts had dominated at Balaidos. Levante are seventh with 21 points.
In the late game, 10-man Real Zaragoza held on for a 1-1 draw at Real Mallorca.
Zaragoza had defender Glenn Loovens sent off for a foul in the area after only 13 minutes, but their keeper Roberto made a great save to deny Victor from the penalty spot.
Two minutes later, Portugal striker Helder Postiga pounced on a loose ball to put the visitors ahead and Mallorca, without a win in 11 matches in all competitions, laboured to react.
Victor finally made amends for his penalty miss with an 86th-minute header for the equaliser.
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