The 24-year-old made 29 appearances in all competitions for Stuttgart during the 2021/22 season, scoring one goal, laying on four assists and earning only four cautions.
A defensive midfielder, Mangala will join the Tricky Trees as their latest signing in what has become a busy transfer window for them.
Having missed the first few games of last season due to a torn muscle bundle, Mangala passed his medical with flying colours.
The Belgian international is currently in Athens where he underwent the medical checks, but is expected to make the trip to London today to put the finishing touches on his transfer, according to Belgian journalist Sacha Tavolieri.
Having made his international debut back in March, Mangala will be hoping to make the Belgium squad for the trip to Qatar in late November for the World Cup.
Forest have returned to the Premier League for the first time since 1999 after beating Huddersfield Town in the playoffs last season.
They will look to establish themselves in the Premier League and hope the squad they are building will be capable of rising to that challenge.
