The track, which was released in January as the second single from Stronger, is now the best-selling single ever by an American Idol contestant, Billboard reports.
The song, which reached number one on the Billboard Hot 100, has surpassed Carrie Underwood's 'Before He Cheats' to achieve the milestone.
'Stronger (What Doesn't Kill You)' has sold 3,510,000 copies as of Thursday (July 5), while 'Before He Cheats' has shifted 3,509,000 units since 2006.
Underwood is still ahead in terms of US album sales. She has sold 13,205,000 albums to date, while Clarkson is close behind with 11,664,000.
Clarkson has achieved digital singles sales of 24,100,000, compared with Underwood's sales total of 22,535,000 digital songs.
Season five's Chris Daughtry and season two's Clay Aiken are the only other Idol contestants to have achieved sales of over five million in the US.
Watch the video for 'Stronger (What Doesn't Kill You)' below: