The 25-year-old former Inverness Caley Thistle striker scored more than 50 goals in his three-year spell in the SPL.
Rooney joins up with former Caley Thistle team-mate Jonny Hayes at Pittodrie, and has been to the Dons squad for Saturday's trip to face Motherwell.
Dons manager Derek McInnes said: "Adam is a proven goalscorer and he has the quality to add something to our team.
"There were other clubs keen to take him and it's great that he wants to be part of what we're doing here at Aberdeen.
"I've said all along that I'll only bring in players who I believe will add something to the squad and Adam certainly ticks that box."