Fiorentina have continued from where they left off in the second half last season, winning just four of their 17 league games.
They failed to win a Serie A match in the last three months of last season and only escaped relegation on the final day of the campaign.
Only a stoppage time equaliser against Inter Milan last weekend prevents La Viola from losing their last six league games.
Montella returned to Fiorentina back in April after impressing in his first stint in charge from 2012-2015 when he guided the club to three straight fourth-place finishes.
His return, however, proved to be anticlimactic as Fiorentina won just six of his 27 games in charge, with two of those victories coming against lower division sides in the Coppa Italia.
“ACF Fiorentina announces that Vincenzo Montella has been relieved of his duties as head coach of the first team,” a club statement read.
“The decision was taken following a long and detailed analysis of the team's performances and results.
“Given the need to rediscover the necessary grit and determination on the pitch and produce an immediate turnaround, the owners and management decided that a change of coach was the best option.
“The club would like to thank Vincenzo Montella for his efforts and commitment in recent months and we wish him every success in the next chapter of his career.
“A new head coach will be announced in the coming days.”
Montella leaves Fiorentina in 15th place, four points above the relegation zone.
