"Happy birthday baby daddy," Kardashian, 37, captioned the photo, which shows her wearing a bikini and straddling Disick.
The pair, who share three kids — Mason, 6, Penelope, 3, and Reign, 17 months — split in July after Disick was spotted cozying up to his ex-girlfriend Chloe Bartoli in the south of France.
He later checked into rehab, and the couple have made an effort to remain cordial for the sake of their kids.
"She'll always be family to me and probably the person I love the most in the world, so even though we're not together, we're still close and it's still nice to be around each other," Disick told Us in March about their friendly relationship.
Kardashian also spoke out about their status that month after fans got excited about seeing the couple spending time together. "Everyone relax," she said in a Snapchat video. "We have three kids together. We spend time together."
Disick remains close to the whole Kardashian clan — momager Kris Jenner was among the family members who wished him a happy birthday on Thursday, thanking Disick "for being a loving and devoted father to my beautiful grandkids."
"I am so blessed to be able to call you my son," she added.
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