Eder Militao vowed to give his all to make history with Real Madrid following his arrival from Porto.
Madrid confirmed in March an agreement was in place to sign Militao after reportedly triggering his €50million release clause.
And following the conclusion of his Copa America commitments with tournament hosts and champions Brazil, the 21-year-old was presented to the media on Wednesday.
Militao, who made one substitute appearance in Brazil, said joining Los Blancos has always been his dream.
"I'm very happy to play for the best club in the world," the youngster said at a news conference that was cut short after he complained of feeling dizzy.
"Since I was a kid I wanted to play for the biggest club in the world and I want to thank the president and all the Madrid fans for their confidence.
"They can rest assured that I will give everything for this jersey. I hope to have a great season here and make history with this club."
Capable of playing at centre-back or right-back, Militao is expected to immediately challenge for a regular spot in Madrid's back four.
"I am a versatile player, who always plays for the love of what he does," he said. "I do not like to lose and I always expect the most from myself. I am very competitive and very competent.
"I think it's going to be a very good fight [for a starting place]. I've come to do my job - it will not be easy, but I'm going to listen to what the coach says."
Militao was the first signing of head coach Zinedine Zidane's second spell and he has since been joined in making the switch to the Spanish capital by Eden Hazard, Luka Jovic and Ferland Mendy.
Madrid president Florentino Perez welcomed the youngster's arrival and described him as one of the best defenders around after seeing off competition from rival clubs.
"Winning is part of this club's DNA and nobody surrenders here," he told reporters. "Today is an important day for us as we welcome one of the great defenders in world football.
"He arrives at just 21, he's adapted quickly to European football and for five months he was part of the best defensive set-up in Portuguese football with Porto.
"His courage and attitude will help him to succeed, as will the millions of fans we have around the world.
"Now you [Militao] will face the biggest challenge of your entire professional career. You arrive at Real Madrid because you have chosen this club ahead of all the big clubs that wanted to have you in their ranks.
"Here you will demonstrate everything you have shown. Your courage and your love for this badge will help you succeed. And our supporters from all over the world will also help you."