The 25-year-old defender, who spent the last half of the previous campaign with the Dons, is glad to have his future sorted out.
"I'm delighted to be returning to Aberdeen. I know the club, the manager and the players well so it's going to be an easy transition for me," he told the club's website.
"I was desperate to get my future resolved and when it was made clear I wouldn't be part of the manager's plans at Sheffield Wednesday, my thoughts turned to possibility of returning to Aberdeen.
"It's great to be back at Pittodrie where I feel wanted and I just can't wait to get my season started."
Reynolds will officially be unveiled as an Aberdeen player tomorrow.