De Rossi, who turns 37 on June 24, retired from a celebrated playing career in January while with Argentinian side Boca Juniors.
The 117-time capped, World Cup-winning Italy international previously made 616 appearances across all competitions for hometown Roma, where he was named Serie A Footballer of the Year in 2009.
De Rossi has made no secret of his desire to enter management like his father Alberto, who is at the helm of Roma's Primavera youth teams.
De Rossi will succeed Giuseppe Iachini at the Stadio Artemio Franchi. Iachini, who replaced Vincenzo Montella on Dec. 23, 2019, guided La Viola to four wins in 14 matches in all competitions and one point from three fixtures following the restart.
Fiorentina currently sit 13th in the table amid a logjam of sides on 31 points, just five points clear of the relegation zone.