The 36-year-old has returned to Old Trafford 12 years after leaving the club to join Real Madrid and will sign a two-year contract.
United posted a collage of Ronaldo from his previous playing days at the club on Twitter along with the words: “Welcome home, @Cristiano.”
A statement from United read “Manchester United is delighted to confirm that the club has reached agreement with Juventus for the transfer of Cristiano Ronaldo, subject to agreement of personal terms, visa and medical.
“Cristiano, a five-time Ballon d’Or winner, has so far won over 30 major trophies during his career, including five Champions League titles, four FIFA Club World Cup, seven league titles in England, Spain and Italy, and the European Championship for his native Portugal.
“In his first spell for Manchester United, he scored 118 goals in 292 games. Everyone at the club looks forward to welcoming Cristiano back to Manchester.”
The Portugal international left for Real Madrid in 2009 for a then world record fee of £80 million but will now make an emotional return.
ESPN reported in June that Ronaldo’s representatives were gauging interest from some of Europe’s top clubs ahead of his potential departure from Juventus this summer after three years in Italy.
Manchester City were one of the clubs who looked at the possibility but on Friday sources told ESPN they had pulled out of the race.
Rio Ferdinand has confirmed he spoke to his former United teammate asking him not to move to the Etihad Stadium.
Talks between Ronaldo, United and Juventus began on Thursday night.
Ronaldo did not train with Juventus on Friday, and instead left Italy on a private jet. Juventus manager Massimiliano Allegri later confirmed he had asked to leave the club before the transfer deadline on Aug. 31.
Ronaldo is one of the most decorated footballers in the world, winning league titles in England, Spain and Italy.
He has won the Champions League five times, including four with Real Madrid, and has won the Ballon d’Or five times. He was first world named world player of the year as a United player in 2008.