United said they had signed the 18-year-old until June 2029. British media reported that United were paying Lille 62 million euros (52.2 million pounds) with 8 million euros in potential add ons.
"Signing for a club with the stature and ambition of Manchester United so early in my career is an incredible honour," Yoro told the Manchester United website.
Yoro made his professional debut for Lille in May 2022 and went on to make 46 Ligue 1 appearances for the club.
"Leny is one of the most exciting young defenders in world football; he possesses every attribute needed to develop into a top-class centre-back," said Dan Ashworth, Manchester United's sporting director.
Olivier Letang, the Lille president, said he was proud of Yoro.
"We're always sad when a boy leaves the club," Letang told the Lille website. "But we're also happy and proud when these boys, who arrived young, become men and top-level footballers."
