Manchester United have announced on their official website that Diogo Dalot has joined the club.
The 19-year-old is the club's second summer signing after Fred.
It is understood that the youngster had a €20million [£17.4million] release clause in his contract.
"I'm very excited for the dimension of this club and I hope to get great things here," he told MUTV.
"I know that this is one of the best clubs in the world and it is unbelievable to be here and I hope to achieve great things.
"I grew up at Porto. Porto gave me everything. Who I am today is because of what Porto gave to me and that has been most important.
"Yeah, it's very good [to be Jose Mourinho's first Portuguese signing at United]. It was very important to make this choice and I hope to help him, because I know that he will help me to improve a lot."
