Facundo Pellistri's move to Manchester United will cost the club €10million.
The 18-year-old became Ole Gunnar Solskjaer 's final signing of the window as he signed a five-year deal from Penarol.
He joins Uruguayan countryman Edinson Cavani in arriving at Old Trafford on deadline day.
United also completed deals for Porto left-back Alex Telles and Atalanta winger Amad Diallo, who will join the club in January.
Highly-rated Pellistri was announced shortly before midnight and insists a Premier League move is a dream come true.
He said: "To join a club with the history of Manchester United is a dream come true. I have learnt a lot at Penarol and I would like to thank them for everything they have done for me. To have the confidence of the manager is an amazing feeling and I know that working with him will be perfect for my development.
“I am coming into a squad full of great players who I will learn from every day. Everyone knows that Manchester United is a club that gives young players an opportunity and I am ready to work hard every day to earn those chances and prove myself in England.”
Despite a late flurry of signings, United missed out on top target Jadon Sancho from Borussia Dortmund.
