McManus is set to be out of contract at the Scottish Premiership club and will join Town on a free transfer on June 10.
The 20-year-old, who has been capped by Scotland Under-21s, will move south of the border on a three-year deal.
McManus was voted PFA Scotland League One player of the year this season after scoring 23 goals for champions Morton during a season-long loan from the Dons.
Everyone at Aberdeen Mad wishes Dec the best of luck in the future!