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.
It looks like an exit of Gareth Bale puts Real Madrid in a very good position to sign other targets due to a huge transfer receipt and freeing of wage bill.
On Thursday they announced the signing of 21-year-old Militao, who will join from Porto in a €50million (£43m) deal in the summer. "I just told you Zidane to Madrid is flawless".
According to WhoScored's data, Militao is a well-rounded centre-back, whose strengths include aerial duels, dribbling, tackling and interceptions.
Militao has scored three goals for Porto this season, including one in the Champions League group-stage win over Schalke.
The Real Madrid midfielder, who is now on a two-year loan at German champions Bayern Munich, has frequently suggested that he is keen to return to his parent club this summer, but Zinedine Zidane's re-appointment as Los Blancos' coach looks to have made that an unlikely eventuality.
The draw for the quarter-finals and semi-finals will take place on Friday.