The 33-year-old Season 1 winner and 33-year-old Season 4 champ will perform on the Fox reality competition's Season 15 finale on April 7. The long-running series will crown a final winner the same evening.
The finale will be preceded by the 90-minute special American Idol: American Dream on April 5. The special will see host Ryan Seacrest, original judges Simon Cowell, Paula Abdul and Randy Jackson, and former contestants share memories of their time on the show.
MacKenzie Bourg, Trent Harmon, Dalton Rapattoni and La'Porsha Renae remain in competition and will be whittled down to three on the March 31 episode. The final three will perform April 6 before the competition is narrowed to two.
Season 2 winner Ruben Studdard, Season 5 contestant Kellie Pickler and Season 6 winner Jordin Sparks are among the other stars slated for the finale. Fantasia, Taylor Hicks, Chris Daughtry and Pia Toscano will also perform.
Clarkson previously returned to American Idol in February, moving the judges to tears with her emotional performance of "Piece by Piece." The song is inspired by her absent father and her love for husband Brandon Blackstock, who fathered her daughter River.
"I definitely didn't know it would affect me like it does when I sing it live because that's not fun," she told On Air with Ryan Seacrest after the performance. "It's awesome, but it's kind of sad that so many relate to this song."