Aina makes a return to London, where he came through Chelsea’s youth system, before moving to Italy initially on loan and later making the move permanent.
The 23-year-old Nigeria international has made 62 appearances in Serie A in the last two seasons.
"I feel very excited and privileged to be able to play for a club like this," Aina told Fulham's website.
"It feels amazing to be back closer to my family, back in the capital where I was born and raised.
"There's a great bunch of lads here. I'm really good friends with Big Hec (Michael Hector) and Josh Onomah.
"I feel very comfortable on either foot, I can fill in wherever needs be. I'd say I'm pretty quick and I like to attack, but I also like to help the team and do the dirty work as much as I can."
Fulham director of football Tony Khan added: "I'm pleased to announce the arrival of Ola Aina to Fulham on loan from Torino. Ola is an exciting young player who can further strengthen our squad this season in the Premier League.
"In our loan agreement, we've secured an option for a permanent transfer for Ola to potentially remain at Fulham if this season proves to be a good fit for us, which is a great situation for the club given our belief in Ola's talent and in his potential. Come on Fulham!"
Fulham will welcome Arsenal to Craven Cottage to kickstart the 2020/2021 season on Saturday.
