Ronaldo scored 118 goals in 292 games during his first spell at United, winning his first Ballon d'Or and Champions League title, as well as three Premier League titles, one FA Cup and two League Cups.
"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. I cannot wait to play at Old Trafford in front of a full stadium and see all the fans again," Ronaldo said in a statement from United.
"I'm looking forward to joining up with the team after the international games, and I hope we have a very successful season ahead."
The five-time Ballon d'Or winner has signed a two-year contract with an option to extend that deal by a further season.
"Ronaldo's return demonstrates the unique appeal of this club and I am absolutely delighted he is coming home to where it all started," said United manager Ole Gunnar Solskjaer, who played alongside the Portuguese during his previous time in England.
Ronaldo is expected to make his second United debut against Newcastle at Old Trafford on 11 September.