The midfielder has signed a five-year contract, until June 2031, after leaving Premier League rivals Aston Villa.
"It's hard to describe how proud I am to join Manchester United. To sign for such a special club feels incredible," says Tielemans.
Last season, Tielemans had seven assists and scored two goals as he helped Aston Villa to the Europa League title and fourth place in the league.
During the World Cup, the 29-year-old scored two goals as Belgium reached the quarter-finals.
