The 26-year-old has initially signed on loan for the rest of the season after which Fiorentina will have an option to buy.
“Krzysztof approached us with the desire for a change,” explained Hertha sports director Fredi Bobic.
“It’s no secret that he feels comfortable in the Italian league, so we found a good solution for all parties involved.”
The Poland striker netted 26 goals in 55 Italian league games for both Genoa and AC Milan, from whom he joined Hertha two years ago.
He had since scored 13 goals with four assists in 58 games for the Berlin club.
Piatek has also netted nine goals in 21 internationals.
He last played in a 2-1 defeat to Hungary in Warsaw in a World Cup qualifier last November which saw Poland finish second in their group and now face Russia in a play-off in March.