After the pop star, 36, performed her new song “Whole Lotta Woman,”she shared a sweet embrace with Cowell, 58, onstage.
“That was bloody fantastic. What can I say?” the former American Idol judge — who was at the show to introduce Camila Cabello’s performance — said of Clarkson’s show-stopping set. “I’m very proud of you.”
Check out PEOPLE’s full Billboard Music Awards coverage to get the latest news on music’s big night.
Clarkson and Cowell, of course, first met on American Idol back in 2002, when Clarkson bested Justin Guarini for the season one title.
Cowell left American Idol in 2010 and launched the U.S. iteration of The X Factor, on which he discovered Cabello when he assembled the girl group Fifth Harmony.
Clarkson — who attended the show with her husband Brandon Blackstock and step-daughter Savannah — was on double duty Sunday at the Billboard Music Awards, where she hosted in addition to performing.
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Since she appeared on American Idol 16 years ago, Clarkson has become one of the most successful artists in mainstream music. She’s released seven full-length albums and a holiday record, as well as a greatest hits compilation. And these days, the Grammy winner sits behind the judge’s table, as she’s a coach on NBC’s The Voice.
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After 15 seasons airing on Fox, American Idol ended in 2015. However, ABC revived the talent competition earlier this year, and it was recently renewed for another season. The season 16 finale airs Monday on ABC.