He had been charged with molesting seven girls - all but one are gymnasts - and at least 140 women are suing him.
Nassar's plea was made as part of a deal with prosecutors. He is expected to receive at least 25 years in prison.
The hearing comes after a third former US Olympian, Gabby Douglas, accused him of sexual abuse.
Ms Douglas, who was one of the so-called Fierce Five that won gold medals at the London Olympics in 2012, revealed her accusation on Instagram.
She offered support for two other Olympic teammates who had already accused the former doctor, adding: "I didn't publicly share my experiences as well as many other things because for years we were conditioned to stay silent and some things were extremely painful."
Douglas reveals abuse by team doctor
Nassar, 54, has already pleaded guilty to multiple charges of child pornography and could face a maximum sentence of life in prison.
Appearing in a Michigan court room, Nassar wore an orange jumpsuit and his hands were kept cuffed to a restraining belt.
As the description of his crimes was read in court, Nassar kept his head lowered.
"The defendant was in a position of authority, and used his authority to coerce the victim", Assistant Attorney General Angela Povilaitis said, adding that several victims were between 13 and 16-years-old.
Many of Nassar's crimes occurred as he was examining the young athletes and at times, happened while the child's parents were nearby.
He has been accused of abusing more than 140 women during his time as team doctor for the US women's gymnastics team. They are currently suing him.
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