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.
This article contains details of an alleged rape.
"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.
"I've just kept it quiet because I've always had something to say, and I'm tired of opening my mouth."
The YouTube docuseries tackles issues in Demi Lovato's life such as trauma, addiction and her relapse into a drug overdose in 2018.
She speaks about her alleged rape saying: "We were hooking up but I said - hey, this is not going any farther.
"And that didn't matter to them, they did it anyways. And I internalised it and I told myself it was my fault because I still went in the room with him."
She says she coped through self-harm, and going through the eating disorder bulimia.
In the series, she also talks about a "promise ring" which was worn by some young stars including herself and the Jonas Brothers, as a commitment to only have sex after marriage.
"So what, I'm supposed to come out to the public after saying I have a promise ring? Six months later, I'm supposed to say, well I had sex, even though it was rape? Some people aren't going to see it that way."
The series also shows the damage Demi's drug overdose had in 2018. She was taken to hospital after being found unconscious at her Los Angeles home.
"My doctors said that I had five to 10 more minutes."
After suffering three strokes and a heart-attack, she's previously said she had been "left with brain damage and I still feel the effects of that".
Those effects include blurry vision that means she can't drive and made reading difficult.
Weeks before the incident, she released a song titled Sober, in which she revealed that after six years of sobriety: "I'm not sober any more."
Demi has previously been open about her struggles with addiction and bulimia, including in her 2017 YouTube Originals documentary Simply Complicated.
She re-launched her singing career at last year's Grammy Awards and sang the national anthem at the 2020 Super Bowl.
In January, she sang on a TV special which marked US President Joe Biden's inauguration.
Demi Lovato: Dancing with the Devil will appear on YouTube from 23 March.
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