The 39-year-old singer sat VIP and cheered on Shelton at the Ball Arena in Denver, Colorado — just weeks after he dumped her estranged husband Brandon Blackstock as his music manager.
She even jumped on the stage, shocking his fans when she began singing with him without warning.
Single Clarkson took her girlfriends to her Voice co-star's concert and documented the whole experience on her social media.
Showing they were "LIVIN’ OUR BEST LIVES," the mom of two wore no makeup for the show and beamed with happiness alongside her besties.
Posing for a selfie, Clarkson proved how laid-back she really is by going barefaced and looked beautiful.
Clarkson continued to be her friend's biggest champion, posting several videos and snaps of Shelton performing and even calling him the "prince of country" music.
But no one was ready for her to sing their duet. Concertgoers caught the whole thing on video, showing Clarkson and Shelton singing his 2014 hit song Lonely Tonight.
"@blakeshelton is KILLIN’ IT tonight in Denver!!! I’ll give him this #PrinceOfCountry 😜❤️ His setlist is INCREDIBLE!!" she wrote.
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Clarkson's public support comes weeks after it was reported Shelton fired her soon-to-be ex-husband as his manager, seemingly taking sides in their ongoing divorce battle.
While the Since U Been Gone singer got Shelton, Blackstock walked away with $200,000 per month and their Montana Ranch, but not for long.
According to reports, Clarkson is gunning to sell the property where Blackstock now calls home.
She gained a huge victory when the judge in their divorce sided with her, agreeing the former couple's prenup would stay in place — which means everything Clarkson made during their marriage remains hers.
Clarkson allegedly bought the ranch, meaning she owns it now and can do what she wants.
When it comes to money, Blackstock is getting plenty from his ex.
She's been ordered to pay him a whopping $200,000 per month, which breaks down to $150,000 in spousal support and $45,601 in child support.
He desperately tried to fight Clarkson over their prenup, citing their drastic income difference.
Blackstock pulls in $10,000 per month as opposed to his estranged wife, who brings in a whopping $1.5 million every month.