The 28-year-old midfielder is currently at the tail-end of a contract with Dundee Utd, but will officially sign for Aberdeen when that expires in July.
New Dons boss Derek McInnes told RedWeb: "I've always been a great admirer of Willo and he's a player I am looking to make a real impact for Aberdeen next season.
"With his extensive SPL experience, he was voted Player of the Year for Dundee United last season which speaks volumes about his ability.
"We are delighted to have him on board as we look towards building a squad capable of bringing success back to this club."