The 38-year-old singer will temporarily replace Cowell as a judge on the NBC talent competition series as Cowell, 60, recovers from a back injury.
Clarkson said Monday on Twitter that she will appear as a guest judge during Tuesday and Wednesday's live shows.
"My friend, Simon Cowell, is doing better now but was in an accident and won't be able to make Tuesday and Wednesday's live shows for AGT, but no worries America, someone far wiser, cooler and hotter is taking his seat! The unbelievably amazing Kelly Clarkson," she teasingly wrote. "You're welcome in advance!"
Cowell's rep said Saturday that Cowell would undergo surgery after injuring his back while riding his new electric bike. Cowell thanked fans and medical staff Sunday on Twitter after his surgery.
"Some good advice...If you buy an electric trail bike, read the manual before you ride it for the first time. I have broken part of my back," the star wrote. "Thank you to everyone for your kind messages."
"And a massive thank you to all the nurses and doctors. Some of the nicest people I have ever met. Stay safe everyone," he added.
Clarkson competed in and won the first season of American Idol, which featured Cowell, Paula Abdul and Randy Jackson as judges. She reunited with the trio on her talk show, The Kelly Clarkson Show, in September.
Clarkson herself has served as a judge on The Voice. She released her eighth studio album, Meaning of Life, in 2017.