The 19-year-old fullback joins Man United a day after the club signed Brazilian star Fred from Shakhtar Donetsk.
Manchester United have announced their second signing in as many days after confirming that teenage defender Diogo Dalot has joined the club from Porto. It comes just 24 hours after the club announced a deal for Shakhtar Donetsk midfielder Fred.
Dalot, tipped to be a long-term replacement for Antonio Valencia, has agreed a five-year contract that will keep him at Old Trafford until 2023, with the option of a further 12 months.
"Joining Manchester United is an unbelievable opportunity for me," Dalot told ManUtd.com.
"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."
Mourinho, who shares the Porto connection with Dalot, said, "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 fullback 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 fullback in Europe and we all believe he has a bright future ahead of him at Manchester United."