Ronaldo penned an open letter to Madrid fans this evening, in which he described his laurel-laden time at Madrid as "possibly the happiest of my life".
"Today Real Madrid wants to give thanks to a player who has demonstrated he is the best in the world and who marked one of the most brilliant periods in the history of our club and world football", the club said in a statement.
Ronaldo departs Real as the club's all-time top scorer, with an extraordinary 451 goals in 438 games.
The five-time Ballon d'Or victor has been Real for nine seasons after joining from Manchester United for a then world-record fee of £80m.
The signing also strikes a blow for the profile of Serie A against La Liga, which has now lost two of its three most famous players in the previous year after Paris St Germain signed Neymar from Barcelona for a world record fee last August.
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. The 13-time Champions League winners believe the Belgian striker is the one to fill Ronaldo's boots in Madrid next season.
He helped the club win four Champions League titles - beating Juventus in the final in 2017 - and also won the league and cup twice each.
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Part of that process will take part on Sunday night, when the starters for each squads are named during a selection show. The Atlanta Braves and Chicago Cubs lead the National League's roster with two starters apiece.
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Pep is committed to playing attacking football, which is a ideal for me, and City's performances last season were outstanding. They walked away from a potential deal for Mahrez in January because they were put off by Leicester's valuation.
Real Madrid confirmed the transfer in a statement on Tuesday evening.
Ronaldo went on to express his gratitude for the time he has spent at the Bernabeu and insisted the club would always have a place in his heart.
Real has not given any details on the financial terms of this transfer.
"It would be great to play with him, but I do not know if there are contacts for the transfer", the midfielder told the press (via Record). "My respect and my recognition to all of them".
He would come back to haunt the Bianconeri on the way to glory in last season's competition, too, with a stunning bicycle kick in the first leg and a nerveless added time penalty in the return. I am leaving but I will always feel that this jersey, this badge and the Santiago Bernabéu are mine, no matter where I am.
"Thanks to all and, of course, as I said that first time in our stadium 9 years ago: Hala Madrid!".