A deal was struck in May 2017 to take the forward from Flamengo to Madrid for a reported £40 million (A$71 million), but only now - with Vinicius Junior having turned 18 this month - has he moved to Spain.
Ronaldo, who starred for both Barcelona and Real Madrid, was alongside Vinicius Junior at his presentation on Friday.
Madrid hope they have signed a South American superstar in the making, despite the 18-year-old only beginning playing senior football in the last 15 months.
"Vinicius is a player with a lot of quality and talent. He's the biggest prospect for the future of Brazilian football. You arrive at an amazing new home. This is where I was the happiest playing football and where I achieved the most important things in my career," Ronaldo said.
"It may be difficult at first, but it's the best place you could be. All players dream of wearing the Real Madrid jersey and now he has the opportunity to show that he has been sacrificing to achieve his dream."
Ronaldo was prolific for Madrid during a four-and-a half-year spell with the club that ended in January 2007, and following the exit of namesake Cristiano Ronaldo to Juventus this summer the onus will fall on others to provide attacking threat.
Vinicius Junior may have to wait for his first-team chance, but he is ambitious and said: "I want to win titles and be successful, giving joy to the Real Madrid fans."
"All the players here are my favourites. I've only seen them on television and video games. Now I'm training with them and that is inexplicable. I have to work hard to get up to the quality they have, he added to realmadrid.com.
Vinicius Junior also said he would love to play with Neymar, the Paris St Germain striker who has been repeatedly linked with Madrid.
But rather than fuel any transfer talk, he stressed that ambition was one he wished to fulfil at international level with Brazil.
"I have a dream of playing with him, and God willing I will be able to play with him in the national team," Madrid's new signing said.