Porto defender Diogo Dalot has completed a move to Manchester United after signing a five-year contract, the club has announced.
The 19-year-old, who reportedly had a £17.4million release clause in his contract, emerged from Porto's academy to make his Primeira Liga debut last season.
United manager Jose Mourinho told the club's official website, "Diogo is an extremely talented young defender with all the qualities to quickly become a great player for this club.
"He has all the attributes that a full-back needs: physicality, tactical intelligence and technical quality, combined with a Porto Academy mentality which prepares players for the maturity they need at the professional level.
"In his age group, he is the best full-back in Europe and we all believe he has a bright future ahead of him at Manchester United."
Portugal Under-21 international Dalot revealed he simply could not turn down the chance to join United and work under compatriot Mourinho.
He said: "Joining Manchester United is an unbelievable opportunity for me. I have grown up in Porto's Academy and I am so thankful for everything they have done for me.
"But the chance of coming to the biggest club in the world is something I just couldn't turn down.
"I'm excited about working with Jose Mourinho and learning everything I can from such a successful coach. I am looking forward to playing alongside the fantastic players in the squad."