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 £17.4m 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, www.manutd.com: "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."