Ronaldo and United reached agreement on a deal on Friday, with the formalities completed on deadline day as the 36-year-old Portuguese sealed an emotional return.
The forward spent six years at United from 2003 to 2009 and returns on a two-year deal with the option to extend for a further year.
“Manchester United is a club that has always had a special place in my heart, and I have been overwhelmed by all the messages I have received since the announcement on Friday,” Ronaldo said.
“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.”
Five-time Ballon d’Or winner Ronaldo won three Premier League titles at United, along with an FA Cup and the 2008 UEFA Champions League, but the club have been success starved in recent seasons.
Ronaldo’s return offers further hope for Red Devils fans desperate for a return to the club’s title-winning days, having not lifted any trophy since 2017.
United boss Ole Gunnar Solskjaer added: “You run out of words to describe Cristiano. He is not only a marvelous 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.”