Ronaldo said he's excited to re-join Man United.
"Manchester United is a club that has always had a special place in my heart," said the Portuguese superstar.
"I have been overwhelmed by all the messages I have received since the [initial] announcement on Friday."
He added: "I cannot wait to play at Old Trafford in front of a full stadium and see all the fans again.
"I'm looking forward to joining up with the team after the international games, and I hope we have a very successful season ahead."
United manager Ole Gunnar Solskjaer said: "You run out of words to describe Cristiano. He is not only a marvellous player, but also a great human being.
"To have the desire and the ability to play at the top level for such a long period requires a very special person.
"I have no doubt that he will continue to impress us all and his experience will be so vital for the younger players in the squad.
"Ronaldo's return demonstrates the unique appeal of this club and I am absolutely delighted he is coming home to where it all started."
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.
Ronaldo was heavily linked with a move to Manchester City but the former Sporting man decided to join Man United instead.
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.
