The 28-year-old Guinea-Bissau international made 113 appearances across all competitions for the Premier League club, scoring 25 goals and establishing himself as a popular figure among supporters.
Beto returns to Italian football three years after departing Udinese, where he recorded 22 goals in 65 appearances.
Italian and British media report that Fiorentina have agreed to pay a total package worth around £15.4 million.