In the summer of 2005, Fiorentina signed Montolivo in a co-ownership deal for €3.5 million. He subsequently signed a five-year contract. He was given the jersey number 18. During the 2005–06 season, Montolivo's first-team chances were limited, partly due to the fact he was still an inexperienced player and partly because manager Cesare Prandelli had a relatively stable and reliable squad of players at his disposal. However, Montolivo's talent did not go unrecognised – he became a regular fixture in the Italy national under-21 team, coached by Pierluigi Casiraghi. Montolivo eventually made his Fiorentina debut in a 2–1 loss to Internazionale.