The two clubs reached a deal reportedly worth 112 million euros (about US$132 million).
The Portuguese star leaves Spain after nine trophy-packed years that saw him score 451 goals in 438 games and win the Champions League four times.
He also wants to complete a remarkable personal record: He has been a European champion six times as senior footballer, with Portugal in 2016; with Manchester United in 2008; with Madrid four times. Their economic commitment represents far more than that, given that Ronaldo will draw a salary of some €30m a year in Turin.
"But I think the time has come to open a new stage in my life and that's why I asked the club to accept to transfer me". This is how I feel and I would ask everyone, in particular our fans, for their understanding...
He went on to thank all his teammates, the staff at the club and most importantly the extraordinary fans, who have been by his side through thick and thin..
He said: "I only have feelings of enormous gratitude for this club and for this city".
"The Fiat workers have made the company's fortune for at least three generations ... and in return they have only ever received a life of misery". Ronaldo has had an astonishing nine years in the Spanish capital.
The Serie A champions' move for the five-time Ballon d'Or victor has prompted a fierce response from the Unione Sindicale di Base (USB), which declared workers at a plant in the south of the country will down tools from July 15 until July 17. And together with them in addition, on an individual level, I have had the satisfaction of having won four Ballons d'Or and three Golden Boots, all during my tenure at this most huge and extraordinary club. All during my time in this vast and extraordinary club.
I suppose I could make this story about the incredible money exchanging hands here. He later played down the comment, but doubts over his future at Madrid lingered, especially after he had also threatened to leave the previous year.