Interviewed in CosmoGirl Magazine, the 2002 American Idol winner said she contracted the eating disorder as a high school freshman, after losing a part in a school musical to a skinnier rival.
"I thought if I came back and I was cuter and thinner - then I'll get the role," Clarkson said. "The lesson I took from that was purely superficial, but that's what I grew up thinking for a long time. It wasn't smart, and I headed straight into an eating disorder and became bulimic for the next six months."
The 25-year-old singer said she recovered when a male friend discovered her problem.
Kelly Clarkson, who noted she's "definitely thicker" than most in the entertainment industry, said Hollywood has a distorted idea of beauty. Her third studio album "My December" arrives June 26 in the United States.