Portuguese full-back Diogo Dalot became Manchester United's second major signing in two days the English giants announced on Wednesday after securing a 22 million euro (£19 million) move from Porto.
Dalot, 19, has signed "a five-year contract, with an option to extend for a further year", United said in a statement.
Brazilian international midfielder Fred joined United from Shakhtar Donetsk for a reported fee of just over £50 million on Tuesday.
United are aiming to close the gap on Manchester City, who won the Premier League by a record 19 points from their local rivals.
Despite only making his senior Porto debut in February, Dalot has been hailed by United manager Jose Mourinho, who made his name as a coach with Porto, as the best full-back in Europe of his age.
“Diogo is an extremely talented young defender with all the qualities to quickly become a great player for this club," Mourinho said in United's statement.
"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's best position is at right-back, although he has at times deputised at left-back, including in a 0-0 draw away to Liverpool in the second leg of the Portuguese champions' Champions League last 16 tie in March.
Full-back has been a problem position for United with Mourinho using converted wingers Ashley Young and Antonio Valencia, both 32, as regulars in his defence over the past year.
"Joining Manchester United is an unbelievable opportunity for me," said Dalot. "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.”
