The centre-half, who spent three seasons at hometown club Forest before leaving for Scotland in the summer of 2011, rejoins the club for an undisclosed fee.
Wilson has signed a three-year contract and leaves Parkhead having won back-to-back titles, and helped Celtic reach the last 16 of last season's Champions League.
The 27-year-old's final appearance for the club came against Elfsborg on Wednesday as Neil Lennon's side progressed to the play-off round of this year's Champions League.
He told the club's website: "It's nice to be back. I'm delighted to get it all sorted and I can't wait to get started to be honest.
"I wouldn't have left Celtic for just anyone, it had to be the right club. And no club fitted the bill better than Nottingham Forest.
"Out of everything, I have probably missed the fans the most. I love playing at the City Ground.
"I know people talk about Barcelona, this game and that game, but I just love matchdays at the City Ground."