Danish striker Rasmus Hojlund completed his £72 million move from Serie A side Atalanta to Manchester United on Saturday.
The forward has signed a five-year deal with the option of a further 12 months.
The 20-year-old, who scored 10 times in 34 games for Atalanta last season, was unveiled to fans at Old Trafford before United's pre-season friendly against French side Lens.
He has scored six goals in six appearances for Denmark.
Hojlund told the club's website: “It is no secret that I have been a fan of this great club since I was a small boy and I dreamt of walking out at Old Trafford as a Manchester United player.
“I am incredibly excited by this opportunity to turn that dream into a reality and I am determined to repay the faith that the club has shown in me.
“It is still early in my career, but I know that I am ready to make this step up and play with this group of world-class players.
"Once I had spoken to the manager, I knew that this environment would be perfect for my development. I am relishing the opportunity to work with one of the best coaches in the world."
His arrival ends manager Erik ten Hag's search for a striker as he looks to reinforce United's front line, with injury-prone Anthony Martial the only recognised centre-forward at the club.
United chief executive John Murtough said: "Rasmus is a truly exceptional talent; he possesses technical and physical attributes which rank him among the best players in the world for his age group.
"Working under Erik ten Hag and his coaches will provide Rasmus with the perfect development platform; everyone across the club will ensure that he is given the time and all the support required to reach his immense potential."
Hojlund began his career at HUI and Brondby before moving to Copenhagen in 2020. In his debut season he scored five goals in 11 Europa Conference League games.
That form earned him a move to Sturm Graz in the Austrian Bundesliga in January 2022, where he scored 12 goals in 21 games, before making a £14.6 million switch to Atalanta.