The Netherlands center half has joined on a big money deal from Bayern Munich.
De Ligt, who has previously worked with coach Erik ten Hag at Ajax, has also played for Juventus in his career.
"I actually really like the project," stated the 25-year-old to club media.
"It's always nice to go somewhere and to feel that you're part of a new project, to feel that you're part of something new.
"Hopefully me, but also the other players that are coming now this summer, can give a great piece to the next history of Manchester United and to get the club back where it belongs: on top.
"Besides that, it's an amazing club, one of the biggest clubs in the world, with an amazing history, a lot of amazing players, amazing defenders...
"I'm really excited. I actually waited a long time! The whole vacation I was waiting for the message, that it was a done deal, and now finally it is. So I can't wait to start."