The club insist the coach's contract extension was agreed "a long time ago" despite speculation over his future this summer.
The Artemio Franchi club confirmed that their 39-year-old coach had penned a new deal during an official squad unveiling on Tuesday, ending speculation over his future.
Though the length of the new agreement was not revealed, Della Valle stated that Montella would remain with the club "for many years".
"This is just a formality, we reached an agreement a long time ago," he said. "I wish to thank the club and the fans for their support, but we are aware that a difficult task is waiting ahead for us now."
Montella arrived from Catania in June 2012 and astutely led his side to fourth in Serie A last season - a huge improvement on its 13th-place finish in the previous campaign.
The Italian has moved to strengthen his squad for the new term, bringing in the likes of Mario Gomez in from Bayern Munich and Josip Ilicic from Palermo.
Fiorentina begin its Serie A campaign with the visit of Catania on Monday.