The 27-year-old rejoins his old club despite making 50 appearances for the Scottish champions last season, admitting that he missed the City Ground and its fans
The 27-year-old made 50 appearances last term and has featured in all of Celtic's Champions League qualifying fixtures so far this season, including Wednesday's 0-0 draw with Elfsborg which sealed an aggregate triumph over the Swedish side.
But now that he has left the Glasgow club, Wilson has revealed that his heart has always belonged at the City Ground.
"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," he told the club's official website.
"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.
"I’ve been at Notts County, Preston and Celtic but looking back and trying to pick a spell I enjoyed the most, even though I played in the Champions League with Celtic, I would have to say it was here.
"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 match days at the City Ground."
The 27-year-old moved to Celtic Park in 2011, having spent four years at hometown club Forest.