The move was announced by the midfielder's former club, Newcastle Jets, on Friday, and the 21-year-old has now put pen to paper on his contract in Florence.
Brillante was part of Ange Postecoglou's initial 30-man squad for the World Cup and, although he did not travel to the finals, he has impressed the powers that be at the Artemio Franchi.
Fiorentina, who finished fourth in Serie A last season, posted a video on their official website showing the youngster completing the formalities of his transfer in Italy.
An accompanying statement read: "Fiorentina announces that it has purchased outright, the rights to the player Joshua Brillante from Newcastle United Jets."
Brillante, who made 46 appearances for Newcastle in the past two seasons, becomes the second young Australian to make a close-season switch to Europe after Adam Taggart joined Fulham.
