Who says it's no fun living in the past?
In the pic, she and another dancer boogie down behind the "Piece by Piece" singer as Clarkson belts out a song.
Though Dewan, 38, didn't say what year the photo was taken, Clarkson's hair is brunette and straight, which is how she wore it for a year or two after winning the first season of "American Idol" in 2002.
"tbt to dancing w/ @kelly_clarkson at a Walmart parking lot in TX," Dewan wrote beside the snap. "I will never forget quick changing in the break room amongst employees, & running through aisles to make it back on stage in time.
"To this day, Kelly is one of THE nicest and best i have met in this business," the "Step Up" star added.
Clarkson, 37, was so tickled by her old pal's throwback, she retweeted it for her fans to see.
"Yes!! What a throwback?! My dream is still to dance as well as you ha! A girl can dream," she wrote, next to laughing emoji.
She added, "We look like such kids in this picture. We need a fresh new pic together girl ha!"
Dewan and Clarkson weren't the only ones with sweet memories of that humble parking lot gig.
Walmart also chimed in — to invite the ladies back!
"Since You’ve Been Gone, we’ve wanted you both back. Jenna and @kelly_clarkson, your move!" the company tweeted.
Dewan's early years as a backup dancer have resulted in other fun, nostalgic moments.
When the former "World of Dance" host visited "The Late Late Show" last July, she and fellow guest Mandy Moore noted that they met on the set of Moore's "So Real" music video nearly two decades ago.
At the time, said Dewan, being in the video was "the biggest deal in the world to me."
"I called my family. I was like, 'I'm doing a Mandy Moore video!'" she recalled, adding, "I was just overly excited. I danced way too hard the entire time."