The 24-year-old Argentinian had been restricted to just three league appearances this term and moves across Tuscany to bolster La Viola's attack, with a view to making the move permanent at the end of the season.
A statement on Viola Channel read: "Fiorentina can confirm acquiring the services of Marcelo Alejandro Paez Larrondo.
"Larrondo, born in Tunuyan on August 16, 1988, arrives on loan with an option to buy him from Siena. He will wear the number 18 shirt."
Larrondo joined Siena in 2009 and played a key role in helping the Bianconeri to promotion from Serie B in the 2010-11 season.