The Premier League giant said an agreement had been reached with Juventus for the 36-year-old, who scored 118 goals in 292 games during his first period at Old Trafford.
“All members of the club look forward to welcoming Cristiano back to Manchester,” read a brief statement from the club.
The five-time Golden Ball winner enjoyed six years of success at Manchester United, winning three league titles and a Champions League, before joining Real Madrid in 2009 for a world record rate .
He is now due to meet with former team-mate Ole Gunnar Solskjaer, who praised Ronaldo on Friday before the deal.
“Cristiano has been a legend of this club, he is a legend of this club, he is the best player of all time, if you ask me,” the United coach said during a press conference.
Earlier on Friday, Juventus coach Massimiliano Allegri said Ronaldo would not play for the Italian club again after saying he wanted to leave.
Reports had linked Ronaldo to a move with Manchester City’s fierce rivals, but a deal never materialized.
Ronaldo won the Serie A title in his first two seasons with Old Lady, but was unable to lead the Italian club to the Champions League title, a competition he won four times with Real Madrid.
Juventus finished fourth last season and speculation about Ronaldo’s future had swirled since then with less than 12 months left on his contract.
“Today I’m leaving an amazing club, the biggest in Italy and probably one of the biggest in all of Europe,” Ronaldo wrote on Instagram after the news of the deal.
“I gave my heart and soul for Juventus and I will always love the city of Turin until my last days.”
He added: “In the end, we can all look back and realize that we have achieved great things, not all that we wanted, but we still wrote a pretty good story together.”
Ronaldo joins Jadon Sancho and former Real Madrid team-mate Raphael Varane as United’s new summer signings, as the club is betting on crawling back to the top of European and national football .
The team has started the season in mixed form, winning their first game against Leeds 5-1 before a disappointing draw against Southampton, and the addition of Ronaldo could give the band a great experience.
The striker is considered one of the best players in history and has won more than 30 major trophies during his career.
During Euro 2020, he matched the all-time international goal record of 109 international goals set by Iranian Ali Daei.
Manchester United captain Harry Maguire wrote a message on Twitter reacting to the news, writing “Welcome home Cristiano”.