The 36-year-old made his name on the world stage in six years at United between 2003 and 2009 before moving to Real Madrid for a then world record transfer fee.
He became a legend at Madrid, scoring a remarkable 450 goals in 438 matches in a glittering nine-year spell that included four Champions League triumphs and winning the Ballon d’Or for the world’s best player four times.
Ronaldo joined Juventus in 2018 and won two Serie A titles, but his time in Turin has not lived up to expectations.
Hopes he would end the Italian giants’ 25-year wait to become champions of Europe have been thwarted.
Juve failed to get beyond the Champions League quarter-finals in the past three years and last season were deposed as Italian champions by fierce rivals Inter Milan.
“Today I depart from an amazing club, the biggest in Italy and surely one of the biggest in all of Europe,” Ronaldo posted alongside a compilation of best moments at Juventus on Instagram.
“The ‘tifosi bianconeri’ always respected me and I tried to thank that respect by fighting for them in every game, every season, every competition.
“In the end, we can all look back and realise that we achieved great things, not all that we wanted, but still, we wrote a pretty beautiful story together.”
United’s swoop capped a tumultuous 24 hours during which Ronaldo initially seemed poised to join rivals Manchester City.
Premier League champions City had been widely linked with a move for the forward after missing out on England captain Harry Kane.
However, United manager Ole Gunnar Solskjaer hinted a deal was in the offing during his pre-match press conference yesterday ahead of tomorrow’s Premier League trip to Wolves.
“Cristiano is a legend of this club, he’s the greatest player of all time, if you ask me,” said Solskjaer, who played alongside Ronaldo.
“We’ve always had a good communication. I know Bruno (Fernandes) has been talking to him as well. He knows what we feel about him. If he was ever going to move from Juventus, he knows that we’re here.”
United announced a deal with Juventus has been agreed with the transfer “subject to agreement of personal terms, visa and medical.”
“Everyone at the club looks forward to welcoming Cristiano back to Manchester,” the Red Devils added in a statement.
Sky Sport Italia reported that the English giants have offered Juve €28 million (B1 billion), agreeing a two-year deal with the superstar attacker worth €25mn per season.
Ronaldo won the first of his five Champions League titles and was first crowned the world’s best player during his first spell at United.
In total he scored 118 goals in 292 games, also winning three Premier League titles, one FA Cup and two League Cups.
“The idea of him going to Manchester City was torture. If there is a big player available Manchester United have to be in the market for that player,” former United captain Gary Neville told Sky Sports.
“They’ve bolstered the squad with a proven goalscorer, a club legend, and what it is going to do is give the club an incredible 12 months.”
Despite missing out on the title to Inter, Ronaldo still topped the Serie A goal charts last season, edging out Romelu Lukaku, who joined Chelsea this month.
And he also rolled back the years at Euro 2020 to win the Golden Boot despite playing just four games as Portugal bowed out at the last 16.
United have already significantly strengthened their squad with the signings of Jadon Sancho and Raphael Varane during the transfer window as they aim to win the Premier League for the first time since 2013.
Ronaldo joins already loaded group of attacking options for Solskjaer featuring Sancho, Bruno Fernandes, Edinson Cavani, Marcus Rashford, Anthony Martial, Mason Greenwood and Paul Pogba.
He is expected to make his second United debut at home to Newcastle on Sept 11.