The Portugal international won four Champions Leagues and two La Liga titles, among nine other major honours, during his nine years at the Bernabeu.
Ronaldo's relationship with president Florentino Perez became fractured, though, and Los Blancos have sanctioned his big-money move to Juve.
Widespread reports suggest that the five-time Ballon d'Or winner will earn £500,000 a week at the Allianz Stadium, taking him through until the age of 37.
The transfer fee has not been disclosed by either club, but it is understood to be worth just over £100m, meaning a profit for Madrid on the £80m paid to sign the 33-year-old from Manchester United in 2009.
A Real statement confirming the news read: "For Real Madrid Cristiano Ronaldo will always be one of our great symbols.
"Real Madrid wants to express its gratitude to a player who has proved to be the best in the world and who has marked one of the brightest times in the history of our club and world football. Real Madrid will always be your home."
Ronaldo scored a club-record 451 goals for Madrid and was crowned the world's best player in 2013, 2014, 2016 and 2017, as well as in 2008 while at Man United.