Vieri, 34, has signed a one-year deal with La Viola. He was available on a Bosman after ending his relationship with Atalanta, for whom he appeared on seven occasions last season, scoring twice.
Vieri's deal reportedly includes incentives depending on how many appearances he makes after he was decimated by injury last season.
"I spoke with him [Vieri] and he's very motivated," Fiorentina coach Cesare Prandelli told La Gazzetta dello Sport.
Serie A preview
The goal-getter has featured for a wide range of Italian clubs throughout his long career, most notably Juventus, Lazio, Internazionale, Milan and Sampdoria.
He has also played for Atletico Madrid and Monaco.
Vieri represents a cut-price option to replace Luca Toni, who has joined Bundesliga outfit Bayern Munich.
Fiorentina qualified for the UEFA Cup with a sixth-placed Serie A finish last season.
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