The 24-year old Northern Ireland international struggled to make an impact with the Hoops following his move from Derry City in 2009 and was subsequently loaned to Brentford last season.
Now he has made a permanent move to Aberdeen and is looking forward to becoming a key player for the Dons under Craig Brown.
"Aberdeen is a massive club and I'm really looking forward to be back playing in Scotland," he said.
"I thoroughly enjoyed my time at Brentford last season and, playing week in week out has, I believe, made me a better player.
"Craig Brown has big plans for the club and I definitely want to be a part of that."