Cristiano Ronaldo says he is "back where I belong" at Manchester United and has dedicated his return to the club to former boss Sir Alex Ferguson.
He has signed a two-year contract, with the option to extend for another year.
"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.