The 31-year-old left AC Milan after six seasons when his contract expired in June, sparking interest in him.
According to Alfredo Pedulla, newly-promoted Benevento offered Bonaventura a better salary than Fiorentina.
However, he has been drawn to the more prestigious Fiorentina, where he will earn €1.7m-per-year plus bonuses.
Bonaventura will sign a two-year deal with the Viola and an announcement should arrive by Monday.
He will bring vast experience to Beppe Iachini’s squad, having made close to 300 appearances in Serie A.
The former Atalanta player has made 13 appearances for the Italian national team since 2013.