The 19-year-old, who has signed a five-year deal, has seemingly been brought in as a direct replacement for Adem Ljajic, who signed for Roma on Wednesday
The 19-year-old has been signed by the Viola on the same day attacker Adem Ljajic joins Roma.
The Italian side announced the purchase of Rebic, who has won one cap for the Croatia national team, on the club's official twitter account on Wednesday, and he will join up with Vincenzo Montella's side immediately.
"Fiorentina announces it has acquired the rights to Ante Rebic on a five-year contract," a statement on the club's official website read. "The Croatian player has already made one appearance and scored once for his country's national team."
Rebic scored 18 goals in 60 appearances for the Croatian side between 2011 and 2013, having joined from the Hajduk Split youth team.
Fiorentina hosts Grasshoppers in its Europa League playoff second leg on Thursday, leading 2-1 from the first leg in Zurich, before visiting Genoa on Sunday in Serie A.