Brillante was part of Australia's extended 30-man squad ahead of the FIFA World Cup and, although he did not get picked to go to Brazil, he has done enough to earn a move to Fiorentina, subject to a medical.
The 21-year-old becomes the second young Newcastle player to head to Europe since the end of last season, after striker Adam Taggart, who did appear at the World Cup, signed with Fulham last month.
And Brillante is thrilled at the prospect of linking up with Vincenzo Montella's Fiorentina.
"I'm very excited to be joining Fiorentina and I'm looking forward to the challenge ahead," Brillante said in a statement released by the Jets.
"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 burst onto the scene with some impressive performances at the 2013 FIFA Under-20 World Cup, including a blistering strike against El Salvador, and he made his senior international debut at the EAFF East Asian Cup last year.
The Queensland-born midfielder leaves the Jets after making 46 appearances over the last two seasons, while he also played 14 A-League matches with former club Gold Coast United.
Brillante won Newcastle's player of the year award at the end of the 2013-14 season.
Newcastle CEO Robbie Middleby said: "This is a fantastic opportunity for Josh to join a club which is competing in one of the toughest leagues in world football and will also playing be in the Europa League next season.
"On behalf of [the] members and everyone at the club, I'd like to thank Josh for his services and wish him all the best with his move to Fiorentina."
