The Seville club are bottom of La Liga and in a desperate need of a goal boost, having scored just 15 times this season - the joint-lowest tally in the league.
Baptistao signed for Atleti in June after an impressive breakthrough season at Rayo Vallecano last term.
The 21-year-old joins the likes of Jorge Molina and Ruben Castro in Betis' striking ranks.
He told the club's official website: "I have signed for a big club with fantastic supporters. I am happy and eager to get to work to help the team in any way I can."
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