Manchester United signs Diogo Dalot from Porto for £17.5m in a 5-year deal.
The Portugal Under 21 international has joined the club for £17.4million and has the option to stay an extra year in his contract.
United wanted a right back to compete with Antonio Valencia, who is 33 and needs constant treatment on a knee injury.
The 19-year-old can also play left back. He broke into Porto’s first team in February in that position and played in his club’s 0-0 Champions League draw at Liverpool.
He helped Portugal win the 2016 UEFA European Under 17 Championship, scoring twice in five games in Azerbaijan including once in the final against Spain.
On signing the teenager, United manager Jose Mourinho told the club’s official website : ‘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 joins Brazilian midfielder Fred, who became United’s first summer signing on Tuesday, at the club.
Dalot 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.