DioGuardi will join the long-running reality show's judicial line-up beginning with its new season on Fox Television this fall, "American Idol" executive producer Simon Fuller said in a news release.
DioGuardi will appear with holdover judges Simon Cowell, Paula Abdul and Randy Jackson in the U.S. version of the show. Its international version has always had four judges.
"We are turning the heat up on 'Idol' this year and are thrilled to welcome Kara to the judges' table," Fuller said. "She is a smart, sassy lady, and one of America's most successful songwriters. We know she will bring a new level of energy and excitement to the show."
DioGuardi's songs have been recorded by such artists as "American Idol" winner Kelly Clarkson, Christina Aguilera, Gwen Stefani, Celine Dion, Faith Hill, Carrie Underwood, Santana and Pink.
"When they told me I was the judge, I just didn't believe it," DioGuardi said. "I still don't believe it. I don't even know what I'm doing here right now."