The former Italy international spent last season on loan at the Stadio Ennio Tardini, featuring 38 times in all competitions.
Having helped the club finish sixth, he completed a permanent move to Parma on Monday - agreeing a deal that will keep him under contract until 2017.
"Parma FC announces that it has acquired outright ACF Fiorentina defender Mattia Cassani," read a statement from the club.
"Mattia, among the protagonists of the extraordinary 2013/2014 season which saw our team finish in sixth place, is available to [coach] Roberto Donadoni and has signed a three-year contract."
Cassani began his career at Juventus before enjoying permanent stints at Verona, Palermo and Fiorentina.
Parma have endured a difficult close-season, with president Tommaso Ghirardi announcing his intention to sell the club following a row over their UEFA license.
Despite that uncertainty, the club have managed to bring in Jonathan Biabiany outright from Sampdoria and now Cassani.
