The 22-year old impressed during Bolton's Championship campaign last season, but he turned down a new contract offer this summer, opting instead for a switch to the Italian giants.
Fiorentina were believed to have been chasing Alonso's signature in the January transfer window, and he revealed his joy at putting pen to paper on a four-year deal despite a number of other offers.
"I am very happy to be at Fiorentina and I hope to help the team to fight for the first positions," he said as he was presented to the media.
"Here at Fiorentina there are great players with whom I can train and learn, like Mario Gomez, Guiseppe Rossi and Massimo Ambrosini - there is a big difference compared to Bolton.
"I had received several offers from England and from Spain, but I've chosen Fiorentina for the confidence they showed in me and for the type of (Vincenzo) Montella's game."
Alonso also admitted that he is focused on claiming a first-team place, although he believes he can learn from current left-back and Fiorentina captain Manuel Pasqual.
"Pasqual? He is a player with many years of experience who can help me a lot, but I hope to play as a regular and I'm not here to be the reserve of anyone," the Spaniard added.