The Viola have confirmed the transfer of the Spanish left back from Bolton on a free transfer
Alonso, 22, who came through the Real Madrid youth team before moving to Bolton in 2010, made 30 appearances for the club last season.
The Spaniard, whose contract was set to expire at the Trotters, will now team up with his compatriot Borja Valero in Florence on a deal which will see him stay at the Viola until 2016.
A statement from Fiorentina on twitter read: "Marcos Alonso is a Viola player!"
Fiorentina is also closely monitoring the progress of Levante No.1 goalkeeper Gustavo Munua.
Vincenzo Montella's men finished fourth in Serie A this season and will be playing Europa League football next year.