The 26-year-old will compete with Mark Reynolds and Ash Taylor for a starting role at centre back for the Dons next season.
Morris spent the last two seasons at Tannadice following a 2014 summer move from Dunfermline Athletic.
After a difficult bedding in period, he would become one of United’s more trusted defenders but struggled to keep his place in the side due to a series of injuries. He played only 15 times last season after suffering two seperate three-month lay-offs.
Morris famously scored against Aberdeen in the 2015 League Cup semi-final, which United won 2-1.