The 30-year-old joins the Reds on a three-year contract worth €9m (£7.6m) and bolsters Steve Cooper’s options in central midfield.
Having joined the Serie A team in 2016 from Luzern, Freuler made 260 appearances for the Bergamo-based club, helping them qualify for the Europa League and Champions League.
Operating on the left of the double pivot in Gian Piero Gasperini’s 3-4-1-2 formation, Freuler excels in his reading of the game and is particularly effective in passing, pressing, turnovers and transitions into attack.
An attacking midfielder, in the number 10 position, is still expected to arrive, with Houssem Aouar, Ruslan Malinovskyi and Morgan Gibbs-White still linked with the role. And the club are also believed to be keen on adding a left wing-back before the transfer window closes on 1 September.
Forest have now spent close to £100m since gaining promotion to the Premier League through the Championship play-offs. Emmanuel Dennis and Cheikhou Kouyaté became the latest signings on Saturday.