The 25-year-old is still to finalise personal terms but, despite being ravaged by injury in recent seasons, his six-year stay at Villarreal looks set to come to an end.
"ACF Fiorentina and Villarreal CF SAD confirm that they have reached an agreement for the transfer of Giuseppe Rossi," a statement on Fiorentina's website read.
Rossi, capped 27 times by Italy, has not played since October 2011 after suffering a knee injury in a match against Real Madrid.
He required two further surgeries on his knee after re-injuring himself in training last April, which ruled him out of Italy's Euro 2012 campaign.
In his absence Villarreal suffered relegation from the Primera Division and, with their star man still sidelined, have opted to sell him on after scoring 82 goals in 192 matches.
A statement from the Yellow Submarine read: "Villarreal want to thank the Italian-American striker for his total dedication in the last six seasons in which he's been at the club, whom he joined in the 2007-08 campaign from Manchester United, and wish him the best of luck in his new stage of his career."
Rossi, who was born in New Jersey, will return to Italy for the first time since a brief loan stint with Parma in 2007 while he was a Manchester United player.
Rossi's arrival will bolster Fiorentina's strike options they look to maintain a Champions League push.
The Viola sit third in the Serie A standings and resume their campaign at home to lowly Pescara on Sunday following the winter break.