He has the option of extending his stay with the club a further 12 months, and according to BBC Sport he cost the Red Devils £19 million.
Dalot told United's official website the move was an "unbelievable opportunity" and that "the chance of coming to the biggest club in the world is something I just couldn't turn down."
Red Devils boss Jose Mourinho added:
"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."
Bleacher Report's Sam Tighe believes he is an intelligent capture from United:
Dalot has left Porto having made just eight senior appearances for the club. Six came in the Primeira Liga, while he also made one outing apiece in the Taca de Portugal and the UEFA Champions League, where he was part of a defence that held Liverpool to a 0-0 draw at Anfield.
Despite his limited number of appearances for Porto, the full-back—who can play on the right or the left—showcased his ability to contribute going forward in that time with three assists.
Portuguese football writer Jan Hagen offered up a profile of the youngster on Twitter:
Samuel Luckhurst of the Manchester Evening News believes his versatility will have appealed to manager Jose Mourinho:
United have needed to strengthen on both flanks, so having a player who can operate on either side is a boost.
Luke Shaw was brought in under Louis van Gaal to become the club's long-term left-back, but he has failed to convince Mourinho, while Matteo Darmian rarely impressed on either flank.
Meanwhile, first-choice options Ashley Young and Antonio Valencia will both turn 33 before the start of next season.
Dalot will look to learn from the pair now that his arrival at the club is confirmed, and United will hope he can soon contribute regularly in the first team as he continues his development.