The 22-year-old enjoyed an impressive spell in Italy during the 2018-19 season, making 30 Serie A appearances - 22 of those starts - while also providing three assists.
The option to sign Aina permanently for £8.7m was included in the deal agreed last summer but facing the possibility of a transfer ban, Chelsea have had second thoughts about selling their talented academy graduate.
The Blues have been closely monitoring their loan players and deciding who is good enough to get a chance with Aina among that clutch of youngsters to be considered for a squad role next season.
But according to Torino president Urbano Cairo, the Nigeria international is now their player.
Speaking to Sky Italia, Cairo said "We exercised our option to buy Ola Aina, as well as [Christian] Ansaldi and [Koffi] Djidji," he said. "Now Torino own all their players."
Aina signed a new four-year contract at Stamford Bridge prior to his loan move to Italy.
