Having been at the London club for over 10 years, 21-year-old defender Aina has impressed in Chelsea's youth system and won two UEFA Youth League crowns and the Under 21 Premier League.
The Nigeria international has made six appearances for the Blues first team, though his last run out was in May last year.
But, after spending last season on loan at Hull City as a regular starter at full-back, he has opted to extended his deal at Chelsea to 2021, but he will have a year in Italy before eyeing a chance in Maurizio Sarri's side .
"Torino have been following me for some time," Aina told the club's official website after his arrival. "I hope to do well; I know this is a club that believes in young players and that is very important.
"I know I can learn a lot in Italy, above all in terms of positioning and organisation, which are fundamental here.
“I really want to experience this new adventure to the full. Thank you to the Toro for choosing me: now it’s up to me and I can’t wait to get started.”
Meanwhile, Torino president Urbano Cairo is excited about the new signing.
"Our staff were already looking for a prospect like Ola Aina, a wing-back who is physically strong, quick and has stamina, able to play on either wing," he said.
“Aina is an ambitious young player who Torino can improve. In the name of the club, I am happy to welcome him to the Granata.”