"He has all the attributes that a fullback 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."
Dalot, who made seven appearances for Porto in all competitions last season, played a part as the club won the Primeira Liga title.
The fullback joined Porto's youth squad, aged nine, in 2008 and came through the ranks to make his first-team debut in February.
"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," said Dalot, who has four international caps for Portugal's under-21 side.
"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."
United did not disclose the financial details of the transaction.
Dalot's arrival follows the news on Tuesday that United have reached an agreement to sign Brazilian midfielder Fred from Ukraine's Shakhtar Donetsk for a reported 52 million pounds ($69.8 million).
(Reporting by Anirban Paul in Bengaluru; Editing by Christian Radnedge, Neville Dalton)
