The Barça striker Samuel Eto'o took his tally for the season to five with two second-half strikes, and Mark van Bommel and Santiago Ezquerro weighed in with their first goals for the club. The victory lifted them into fifth place in La Liga, three points adrift of the leaders.
Barcelona, who rested both Ronaldinho and Deco, were handed an early chance to take the lead when awarded a penalty after Henrik Larsson was brought down by David Rivas. However, Eto'o hit the post with his penalty-kick.
They went in front, though, in the 20th minute when Dutch midfielder Van Bommel, in the starting line-up for the first time this season, headed past goalkeeper Toni Doblas from a Xavi free-kick.
Betis suffered another setback almost immediately when the striker Ricardo Oliveira was sent off for insulting the referee, who had waved play on after the Brazilian appealed for a free-kick.
The home side still managed to equalise before half-time, however, when defender Juanito got in between two defenders to steer a header past the Barcelona goalkeeper Victor Valdes from a Marcos Assuncao free-kick.
Barça eventually took advantage of their extra man early in the second half, Eto'o hammering the ball into the roof of the net after substitute Andres Iniesta had slipped a neat pass into his path.
The Cameroon striker fully atoned for his earlier penalty miss when he grabbed a second, drilling home from close range after stealing the ball off the toe of team-mate Maxi Lopez.
Ezquerro got the visitors' fourth with a pin-point finish from 20 yards out two minutes from time.
Bernd Schuster's Getafe returned to the top of La Liga after beating neighbours Atletico Madrid 1-0 at the Vicente Calderon stadium despite being reduced to 10 men early in the second half.
The midfielder Mario Cotelo was sent off on the hour for a wild challenge on winger Martin Petrov.
But the Argentinian defender Mariano Pernia struck the winner 10 minutes later when he fired a free-kick through a ragged Atletico wall.
The victory left Getafe leading the standings with 11 points from five games.
Valencia moved up to third with a 2-1 win at home to Real Sociedad, striker David Villa joining Eto'o at the top of the scoring charts after hitting his fifth of the season at the start of the second half.
Pablo Aimar had given Valencia the lead in the first half, while Nihat Kahveci struck to pull one back for Real Sociedad in the 62nd minute.
Villarreal finally recorded their first victory of the season and moved off the bottom after a 2-1 win at Espanyol, courtesy of goals from the midfielders Josico and Marcos Senna.
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