Announcing the arrival of the 26 year old, Pittodrie legend and Director of Football Willie Miller told RedWeb: "It's no secret that we've been searching to bring additional quality and skill to the squad and we've identified certain targets. However, we also don't have an open cheque book and we have to ensure the funds we do have are utilised wisely and on players we are convinced will improve the quality of the overall squad.
"It is also crucial that the players we do sign are absolutely committed to playing for Aberdeen and in Barry's case, right from the outset, he made it clear he wanted to come to the Dons. He's a top quality player, a Scottish internationalist and I'm delighted he's agreed to join us."
The Dumfries born player, who passed a medical this morning despite missing the latter part of last season to undergo a hernia operation, said "I'm really excited at the prospect of being reunited with Jimmy Calderwood and Jimmy Nicholl and the fact things took a wee while to get sorted out was absolutely nothing to do with Aberdeen. This is a huge club and a fantastic move for me. My recent operation was a complete success and I should be fit and ready for the start of pre season at the end of the month along with the rest of the lads."
Nicholson, who has played three times for Scotland, was with Dunfermline for five years, having joined from rangers in 2000. He has captained the Pars on several occasions in the absence of regular skipper Scott Thomson, and was central to the Fife club's surprising performance last season which saw them finish fourth in the SPL, get to the Scottish Cup final and qualify for Europe.