After 12-years since departing the club, the Portuguese forward has now returned and is expected to feature for the Red Devils after international break.
Commenting on his re-union with the Old Tarfford club, Ronaldo dedicated his return to Manchester United to former boss Sir Alex Ferguson, in a post on his Instagram account.
"Everyone who knows me, knows about my never-ending love for Manchester United," he wrote on Instagram.
"The years I spent in this club were absolutely amazing and the path we've made together is written in gold letters in the history of this great and amazing institution.
"I can't even start to explain my feelings right now, as I see my return to Old Trafford announced worldwide.
"It's like a dream come true, after all the times that I went back to play against Manchester United, and even as an opponent, to have always felt such love and respect from the supporters in the stands.
"This is absolutely 100% the stuff that dreams are made of!
"I'm right here! I'm back where I belong! Let's make it happen once again! PS - Sir Alex, this one is for you…"
Ronaldo has joined Man United on two-year contract, with the option to extend for another year.
Spending six years at United, Ronaldo won three Premier League titles, one Champions League, two League Cups and one FA Cup - as well as the Fifa Club World Cup and Community Shield - under Sir Alex Ferguson.
The five-time Ballon d'Or winner scored 101 goals and provided 22 assists in 134 games in all competitions during his three-year stay in Turin.
