The 25-year-old Irishman arrives from Oldham and goes straight into the squad for the Scottish Premiership trip to Motherwell at the weekend.
Dons boss 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."
The Dublin-born player joined Inverness in the summer of 2008 before leaving for Birmingham at the start of the 2011-12 campaign.
A season-long loan at Swindon followed a year later before he joined Oldham last summer.