The 36-year-old's departure from Juventus was confirmed by boss Max Allegri on Friday, as he told a press conference he was told by the player he had "no intention" of playing for the club again.
Cristiano, a five-time Ballon d’Or winner, has so far won over 30 major trophies during his career, including five UEFA Champions League titles, four FIFA Club World Cups, 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.