The 22-year-old had an impressive one-year loan spell at the Serie A club, making 32 appearances and scoring once.
Torino president Urbano Cairo announced that they had exercised their option to buy Aina with the current deal set to expire at the end of June.
We’ve redeemed Aina for €10m, as well as Cristian Ansaldi, and Koffi Djidji. Now Torino own all their players.
Meanwhile, Torino finished seventh on the table.