Reynolds has put pen to paper on a four-and- a-half-year deal at the club after having his contract with Sheffield Wednesday terminated.
The 25-year-old, who spent 5 years at Motherwell, was on loan with the Dons from the Yorkshire club for the last calendar year.
Reynolds said: "After a spell of uncertainty, it's great to get my future sorted out and know that I'm going to be in the north east for the foreseeable future.
"My family settled in Aberdeen very quickly and we really like the area and, with the recent birth of our son Lucas, it's a huge boost to know that we'll be staying here.
"It's up to me to repay the faith the club has shown in me and I do believe we have a squad capable of bringing success to Aberdeen."