The 40-year-old nabbed the black crop top with fake bone detailing and lace-up black pants from her 1993 "If" music video.
"Happy birthday queen!" Kardashian wrote on a since-removed Instagram Story with a clip of Jackson in the outfit. "For @janetjackson's bday bc I'm such a fan I can't believe I won this on juliens_auctions."
"Thank u so much @kimkardashian! I hope 'IF' gives u as much pleasure as it did me," Jackson responded.
The outfit sold for $25,000 as part of the three-day "Iconic Treasures From the Legendary Career and Life of Janet Jackson" action to commemorate Jackson's birthday.
"Can't believe this will be the last time I'll see some of these items, but it's for a good cause," Jackson said earlier this month when she announced the sale.
"This was the best outfit I’d ever seen up to that point & maybe still is," one fan noted when the outfit sold.
"Congrats to Kim K. For the win!!!" a second user said.
"This outfit is so iconic!" a third gushed.
Jackson also shared a moment from the auction when fans sang "Happy Birthday" as she virtually tuned in. "Thank u guys for all the birthday luv!" she gushed. "I luv u so much 😘😘😘."
According to Page Six, Kim even dated Michael's nephew T.J. Jackson as a teen.