Aina joins as a free agent after his contract with Torino expired.
The 26-year-old made 113 appearances for the Serie A side across his two spells at the club, with his first coming on loan while on the books of Chelsea in the 2018/2019 season.
Aina spent the 2020/2021 season on loan at Fulham from Torino.
The Nigeria international becomes Nottingham Forest's first signing of the summer transfer window.
“It's exciting to be back playing in the Premier League. It's the best league in the world and I always want to challenge myself,” Aina told the club’s official website.
"The manager as well, what he wants from the team is something which I want to buy into. He's told me it's a family club., he's told me we have a great bunch of players and he's told me I'll enjoy it here.
“I know some of the boys well already which is good for me, but I'm excited to meet the rest of my team-mates and get to know everyone.”
Head coach Steve Cooper added: “He's a versatile player who has good experience of the Premier League and Serie A. He's a really good player, but a good athlete as well.
"He'll definitely make the team stronger and we're looking forward to working with him.”
