The club were able to register one player ahead of the UEFA deadline tonight, allowing Mackay-Steven to play in the club’s Europa League clash against NK Siroki Brijeg tomorrow.
The Thurso-born player, 26, has also played for Ross County, Liverpool and Airdrie United. He joined Celtic in 2015.
A spokesman for the Dons said the club is “delighted” with the signing.
He added: “The wide player, who will be given squad number 11, becomes Derek McInnes’ fifth signing of the summer so far.
“He will join fellow new boys Greg Tansey, Ryan Christie, Greg Stewart and Nicky Maynard in the dressing room.
“Everyone at Pittodrie would like to wish Gary all the very best during his AFC career.”