Ellen DeGeneres reportedly bullied an 11-year-old boy, calling him "fat" and "stupid," made fun of his clothes and "took pleasure" in his distress.
The now-grown victim Ben Gravolet claims the TV host worked for his mother's recruitment agency when he was a child and she was aged 20, and says she would relentlessly abuse him.
"I would dread going to [my mom's] office to see her after school or on a day if I was sick and Ellen was there," Gravolet told The Daily Mail.
"She would criticize my weight. I would try to do homework in the office, she'd call me stupid, she'd call me fat. She would criticize my clothes."
"I was just a boy and this was a grown woman who took pleasure in seeing me become visibly upset."
The 52-year-old says the comedian "was just the meanest, nastiest, most horrible person," and that her abuse has had a lasting effect on him.
"It has an effect if somebody in a superior position to you, who's much older, goes ''You're fat. You might want to lose some of that weight chunky boy.'' I was always self conscious about my weight and it made things worse."
"As I watched her meteoric rise to fame, people would say how great she was and all I could think was she must be an incredible actor because she was one of the most vile people I've ever met in my life,' he said. 'Who takes pleasure in giving a child pain?"
Producers of The Ellen Degeneres Show have been accused of creating a "toxic work environment" by several former employees, as well as sexual misconduct claims being placed against two of the show's top executives.
While Degeneres, 62, was not accused of any wrongdoing in the allegations, she sent a letter of apology to her staff last week, which caused backlash after Brad Garrett - who appeared on the chat show six times between 2004 and 2007 - accused her of treating some people on the programme "horribly".
A number of celebrities - including the likes of Katy Perry and Kevin Hart - have spoken out to defend her after Everybody Loves Raymond star Garrett slammed her on Twitter.
The 60-year-old actor wrote: "Sorry but it comes from the top @TheEllenShow. Know more than one who were treated horribly by her. Common knowledge."
Meanwhile, in her letter to staff, Degeneres said she was "disappointed" to hear about the allegations surrounding her show's producers.
She wrote: "On day one of our show, I told everyone in our first meeting that The Ellen DeGeneres Show would be a place of happiness - no one would ever raise their voice, and everyone would be treated with respect. Obviously, something changed, and I am disappointed to learn that this has not been the case.
"For that, I am sorry. Anyone who knows me knows it's the opposite of what I believe and what I hoped for our show."