Cunningham has signed a four-year contract and joins Bristol City for an undisclosed fee.
Bristol City boss Derek McInnes told the club’s official website: “The left back position is one that we’ve needed to strengthen and fill.
“Greg is someone who has interested us for a long time. We scouted Nottingham Forest a number of times at the back end of last season and he really caught the eye.
“He’s a good player who is only going to improve. Greg is a solid defender, determined and full of energy and pace.
“We had a number of options put to us in that position but good left-backs are hard to find and Greg was always the one we wanted. I believe we have secured the services of one of the best up-and-coming left backs
“It’s a great financial deal for the club and we get an excellent player out of it on a long-term, secure contract.”