The Portuguese full-back has signed a five-year contract with a club option of a further year, and becomes United’s second addition of the summer transfer window after an agreement was reached to sign Fred from Shakhtar Donetsk .
Dalot only made seven senior appearances for Porto but had previously been a regular for their B team and has also played for his national team at every level up to under-21s.
He will now add much-needed depth and competition to United’s defensive stocks, with Jose Mourinho having been keen to add extra options beyond 33-year-old first-choice full-backs Antonio Valencia and Ashley Young.
Mourinho said of his new signing in a club release: "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.”
Dalot added: "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."
Dalot’s signing comes at a time when Matteo Darmian looks set to leave for Juventus and Daley Blind has also been linked with an exit. Luke Shaw’s future is also the subject of much speculation following the England left-backs slide down the full-back pecking order.
The youngster was featured in Goal 's NxGn XI earlier in 2018, featuring as right-back in the squad made up of the greatest teenagers in the game.