Montella has signed a two-year contract with the Tuscan club with an option for a third year.
"Fiorentina convinced me [to sign] because they want to be great again, to return to the part of the league table where they belong, and I feel the same desire," the former Roma striker told a news conference.
"The club have shown me great respect and this motivates a great feeling of responsibility in me because Florence is a great place."
"The club have asked me to recreate enthusiasm for the team, we want to bring ourselves closer to the supporters," he said.
Montella replaces Delio Rossi, who was sacked towards the end of last season after punching his own player Adem Ljajic during Fiorentina's 2-2 draw with relegation rivals Novara.
He had an impressive season with Catania, guiding them to a comfortable mid-table finish and playing an exciting brand of football in the process, and he had been linked with the AS Roma job after Luis Enrique's exit.
Catania announced the signing of Rolando Maran, who lost the Serie B playoff final to Sampdoria with Varese this weekend, as Montella's replacement on Monday.
