Madrid have secured the once-capped Brazil player on a six-year contract, with the club reported to have triggered his €50 million (A$80 million) release clause.
The Spanish giants announced the transfer on their official website this morning.
Interestingly, the centre-half, who also doubles as a right-back, featured 20 times for Porto in the Portuguese topflight this campaign, netting two goals along with two assists.
It's clear that change is coming for Real Madrid.
Real said Militao would join on a four-year deal until 2025.
"He comes to the Whites from Porto, where he has quickly become a star of the team in only his first year in the Portuguese League".
Former Madrid defender Pepe - who plays alongside Militao at Porto - is likely to be a good source of advice for the youngster, having left the Dragons in 2007 to spend the next decade at the Santiago Bernabeu.
He is also a full Brazil worldwide and made his debut against El Salvador in September 2018.
"He stands out for his speed, positioning and ability to anticipate danger. Qualities that he has worn since his arrival last summer at Oporto, where he has shown a rapid adaptation to European football".
And he hasn't wasted any time in making his first addition.
"He's a key piece of the meanest defence in Portugal, and has been voted the championship's best defender in September, October, November, December and January".