The 21-year-old Australian international was named the club’s player of the year this past season, and is adept at playing as a holding midfielder or a right back.
Brillante confirmed the move on the Newcastle Jets’ official website.
“I’m very excited to be joining Fiorentina and I’m looking forward to the challenge ahead,” the statement read.
“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.
“Newcastle will always be a part of me now and I’ll be keeping an eye on the Jets and hoping the club does well in the future.”
Fiorentina reportedly beat Lazio and Everton to the services of the versatile midfielder.