The tall centre-back has signed a four-year contract with Betis.
Pezzella being at Fiorentina since 2017 made 138 competitive appearances in the Viola jersey, with seven goals and six assists.
It is the 30-year-old's second spell with Betis as he had already played for the side from 2015-17.
Pezzella was part of the Argentina squad that won the Copa America over the summer, beating Brazil 1-0 in the final in Rio de Janeiro.
Real Betis finished sixth in the league last season, booking a place in the Europa League and will hope to do better than they did last term.
Betis announced his arrival on their social media with a video based on Grand Theft Auto.
Pezzella, aside playing for both Real Betis and Fiorentina has also featured for River Plate in Argentina.
Pezzella's first stint with Real Betis was in 2015, joining the club from River Plate when Los Verdiblancos had gained promotion to the top-flight.
In thirty-two Serie A games, Pezzella scored just one for the Viola last season, helping them finished 13th in the league log.
The former River Plate centre-back won't be available for selection in Betis' second game of the season against Cadiz at the Estadio Benito Villamarín as they are yet to record a win after sharing spoils with Real Mallorca.
Meanwhile Fiorentina who will be without their captain Pezzella for the new Serie A season will be hosted by AS Roma for the season opener.
