As announced on Forest’s official website, McKenna has joined the Championshipclub in England for an undisclosed fee.
The 23-year-old defender came through the Dons’ academy and has established himself as an important player for the team in recent years.
After loan spells at Ayr United and Alloa Athletic, the Scotland international became an integral part of Derek McInness’s side and has one of the best defenders in the Scottish Premiership in recent years.
Forest are aiming to finish in the top six of the Championship table this season, but the Reds have not started the campaign strongly.
Sabri Lamouchi’s side have lost both of their opening games of the campaign and have conceded four goals.
Forest will return to action on Friday evening when they take on Huddersfield Town away from home in the Championship