A product of Inter's youth system, the left-back has joined on an initial loan deal with an option to buy at the end of this season.
Brazilian defender Dalbert has moved in the opposite direction, also on loan.
Biraghi, 26, will hope it is a case of third time lucky for him at San Siro, having left to join Cittadella and Pescara in 2012 and 2016 respectively.
He penned a four-year deal on his return to Inter from Cittadella, on the other side of a loan to Catania, in 2014 but was handed temporary spells at Chievo and Granada before being moved on again.
Impressive displays in Pescara's ultimately failed battle against Serie A relegation in 2016-17 caught the eye of Fiorentina and Biraghi has chalked up 70 top-flight appearances over the past two seasons.
With Fiorentina dropping from eighth in 2017-18 to 16th last season, Biraghi will be hoping his return home will see him fight further toward the top end of the Italian table.
Inter won their first competitive game under Antonio Conte 4-0 against Leece on Monday, with record signing Romelu Lukaku and fellow new arrival Stefano Sensi on target.
Conte's Inter side looked hugely promising against their newly promoted opponents, with stunning goals from Marcelo Brozovic and Antonio Candreva bookending an assured team performance.
Inter are hoping Conte can be the man to turn them back into title contenders, though the new boss has admitted there is a significant gap to bridge between themselves and the top two Juventus and Napoli.
Biraghi's arrival continues a busy transfer window for the Nerazzurri, with the club expected to add Alexis Sanchez on loan from Manchester United.
Inter's next fixture sees them travel to Cagliari on Sunday.