The 22-year-old defender made 30 league appearances as Torino finished seventh, scoring his first goal for the club in a 1-0 win over Udinese in February.
An FA Youth Cup and UEFA Youth League winner during his time at the Chelsea academy, Aina made just three Premier League appearances for the London side, all of which came in his second season as a senior professional as Antonio Conte guided them to the Premier League title.
He was loaned out to Hull City during Conte's second season at the club and was a regular starter for the Tigers, making 44 appearances. It was during that spell that he was called up to earn his first full cap for Nigeria.
Thanks @Aina2Ola , and good luck for the future!— Chelsea FC (@ChelseaFC) June 11, 2019
Torino have opted to take up their option to sign Aina at the end of his loan spell, as president Urbano Cairo confirmed recently, and will pay a reported €10 million (£9m/$11m) to land the right-back, who is preparing to represent Nigeria at the Africa Cup of Nations this summer.
A statement on Chelsea's official website confirmed Aina is set to leave the club he joined at the age of 11.
"Everyone at Chelsea Football Club would like to thank Ola for his services and we wish him the best of luck for the next stage in his career," it read.
Aina's departure means Cesar Azpilicueta and David Zappacosta are the only right-backs in the Chelsea squad. As the club are banned from signing players in the next two transfer windows, 19-year-old Reece James is expected to compete for a starting spot after an impressive season on loan at Wigan.
Meanwhile, other young players returning from loan spells, such as Tammy Abraham, Kurt Zouma and Mason Mount, could also play a role in the Stamford Bridge side's campaign.
Chelsea will at least see one fresh face arrive this summer in Christian Pulisic, who signed in January but finished the season on loan at Borussia Dortmund.