The former Atletico Madrid youngster, who has penned a four-year deal with the La Liga side, hailed the move as an "important step" in his career.
"I am happy that a club like Betis came calling," he said. "This move represents an important step in my career.
"This club had a good campaign last season and I am now looking forward to the possibility of playing in the UEFA Europa League."
Betis finished in seventh place in La Liga last season.