The Australian international, who also holds Italian citizenship, impressed during the A-League last season, and the player, who leaves Newcastle Jets after making 46 appearances, is thankful for the opportunity given to him by the New South Wales outfit.
The 21-year-old told the Jets' official website: "I'm very excited to be joining Fiorentina and I'm looking forward to the challenge ahead.
"I've had an amazing couple of years at the Jets and I'm very thankful that I came to a club like Newcastle who develop young players and give them an opportunity.
"Having the chance to play regular first-team football has helped my game improve a lot and it is a big reason why I've been able to achieve things like playing for the Australian national team."
Brillante was named in Australia's preliminary squad ahead of the World Cup finals in Brazil before he was one of seven players dropped from the camp.