The 28-year-old revealed in her documentary Demi Lovato: Dancing with the Devil that the person faced no consequences after she came forward.
The singer does not say who the offender was, only that she "had to see this person all the time" afterwards.
Radio 1 Newsbeat has contacted the Disney Channel for comment.
The documentary was shown at the South by Southwest virtual festival.
"My MeToo story is me telling somebody that someone did this to me and they never got in trouble for it," says Demi Lovato in it, according to Variety's review.