That fee, paid in a single lump sum, represents an incredible profit for United, who bought Ronaldo from Sporting Lisbon for £12 million in 2003.
Ronaldo became recognised as the world's best player during his time at United, helping the club to three successive Premier League titles and winning the Champions League in 2008.
'Cristiano has been a marvellous player for Manchester United," said Sir Alex Ferguson. "His six years at Old Trafford have seen him develop into the best footballer in the world.
"His contribution has been a major factor in the club's success in that time and his talent, his ability to entertain and his infectious personality have enthralled fans the world over.
'Everyone here wishes him well in his future career.'
Ferguson famously described Real as "that mob" and claimed he "would not sell [them] a virus". And he has yet to offer a full explanation about his apparent change of heart.
Ronaldo will be unveiled at the Bernabeu next week, and it has yet to be decided what number he will wear at his new club. Ronaldo's favoured No 7 shirt is occupied by Madrid captain Raul, although the new arrivals have cast doubt on Madrid legend 's future.