The 19-year-old defender, who is understood to have cost in the region of £19m, has signed a five-year deal that also includes a further 12-month option at Old Trafford.
Diogo Dalot admitted he could not turn down the chance to join “the biggest club in the world” after completing his move to Manchester United from Porto.
He joins forces with compatriot Jose Mourinho, who rates the player as “the best full-back in Europe” in his age group.
Dalot said: “Joining Manchester United is an unbelievable opportunity. I have grown up in Porto’s academy and I am so thankful for everything.
“But the chance of coming to the biggest club in the world is something I 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.”
Dalot only made his Primeira Liga debut in February, but helped Portugal win the European Under-17s European Championships in 2016 and is a current Under-21 international.
Mourinho is expecting big things of Dalot during his time in England.
The manager 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 a full-back needs: physicality, tactical intelligence and technical quality, combined with a Porto academy mentality.
“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.”