The 36-year-old, who spent six years at United between 2003 and 2009, officially re-joined the Premier League side on Friday. Sources tell ESPN that the deal is worth €15 million with a potential further €8m in add-ons.
A team statement 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."
Manchester City had seemed to be Ronaldo's next destination after ESPN sources revealed on Thursday that the former Real Madrid player had spoken on the phone to City manager Pep Guardiola about moving to the Etihad.
But City abandoned their attempts to sign Ronaldo, who had become a target following their failure earlier this week to strike a deal with Tottenham for Harry Kane. United then opened talks with Ronaldo's agent Jorge Mendes on Thursday.
Sources have said, however, that Ronaldo still favoured a move to Premier League champions City when United entered negotiations and was only convinced to consider a return to Old Trafford following contact from a number of former teammates.
Sources have told ESPN that after being made aware of United's late interest, it was communicated to Juventus that he wanted to return to Old Trafford and asked the Serie A side to open negotiations.
Swap deals with players moving to Turin were discussed but sources have told ESPN United will instead pay a transfer fee. Plans are in place for Ronaldo to undergo a medical in Lisbon this weekend after he left Italy on a private jet on Friday.
"I rang him [Ronaldo] straight away: 'What's going on?'" United legend Rio Ferdinand said. "Every type of no, no, no in the conversation.
"I'm like all the fans, I'm exactly the same. We're all the same. 'Please tell me you're not going there.'
"I hope there's an announcement at some point and it's one that I would be happy with."
City were left unhappy by Mendes' behaviour, according to ESPN sources, as Ronaldo's agent decided it would be better for the player to make an emotional return to Old Trafford rather than join rivals City.
ESPN has been told that Ronaldo was still exchanging texts and WhatsApp messages with some ex-United players as late as 1 a.m. BST on Friday morning.
Wayne Rooney, who played alongside Ronaldo at United, spoke on Thursday to say he doubted that Ronaldo would risk his reputation at Old Trafford by signing for City.
And sources have told ESPN that Ronaldo was told in clear terms by former teammates that his standing in United's history, and the affection with which he is still held by the fans, would be damaged forever if he signed for City.
Sources added that the contact from ex teammates tugged at Ronaldo's heart strings to the extent that he informed Mendes he favoured a move back to United rather then City, leading to the Etihad outfit backing out on Friday to leave United a clear path to sealing a deal.
Information from ESPN correspondent Rodrigo Faez was used in this report