Militao has signed a six-year deal at the Bernabeu, with Madrid confirming he has committed his future to the club until 2025.
The 21-year-old central defender is comfortable at right-back and in midfield and has been courted by a host of Europe's elite but Los Blancos have acted swiftly to secure the signature of the highly-rated Brazilian talent.
Real Madrid have had many issues during the 2018/19 season, one of which has been the defence. It stands out for its speed, placement and ability to anticipate.More news: British PM Theresa May must chip away Brexit resistance bloc by bloc
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He has scored three goals and picked up two assists in 34 appearances for the Portuguese side, with his displays catching the eye of Madrid.
Porto acquired him from Sao Paulo and handed him a five-year contract. His debut came against Moreirense in the League and he got his name on the scoresheet for the first time against Schalke in the Champions League.
"His superb performances have led to him making his full Brazil global debut in a clash with El Salvador in September 2018".
Militao only arrived in European football during the summer of 2018.