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.
Seven goals in 28 games were enough to convince Diego Simeone to bring the Brazilian-born striker across Madrid, although he has only managed five appearances in all competitions - scoring once.
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."
Betis take on Osasuna at home on Sunday, looking for just their third Liga win of the season.
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