Ronaldo left Manchester United to join Real Madrid in 2009, and won four C.L titles with the Spanish giants.
During his time in Madrid, the five-time Ballon d'Or recipient broke a host of records while donning the famous white strip of Los Blancos, including becoming the club's all-time top goalscorer, breaking Raul's previous record three years ago.
Ronaldo will be tasked with bringing Champions League glory back to Juve, who have been European runners-up in two of the past four seasons despite winning their domestic league for seven straight years.
These years at Real Madrid and in this city of Real Madrid have quite possibly be the happiest years of my life. I only have feelings of enormous gratitude for this club, for this hobby and for this city. I have felt the warmth of an incredible fan base and together we have conquered three straight Champions Leagues titles and four Champions League titles over five years.
Real Madridconfirmed the transfer late on Tuesday afternoon.
Cristiano Ronaldo is the most recognizable star in the world but fans are going to have to get used to him wearing a new jersey.
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Earlier, Real Madrid had omitted Ronaldo from their new kit campaign.
Juventus president Andrea Agnelli flew out on Tuesday (Wednesday NZ Time) to meet the five-time Ballon d'Or victor. "You have earned a special place in the history of Real Madrid".
Ronaldo will rightly be remembered as one of Real's best ever players, although he never quite enjoyed the God-like status he craved.
The Spanish club said in a statement that it was because of Ronaldo's intentions and desire to leave that they accepted the deal, but that Madrid will always be his home.
He most recently lead Portugal to the Round of 16 at the 2018 World Cup, where they were beaten by Uruguay.
While Juventus clean up domestic honours year in year out, the addition of Ronaldo will bolster their chances of raising the prestigious Champions League trophy.