The actress split from Brian Austin Green in May.
In the sexy black-and-white mirror selfie, Kelly sports only a towel around his waist, while Fox wears the same, with a bikini top.
"Achingly Beautiful Boy... My heart is yours ?♥️?," Fox captioned the pic.
Fox also supported Kelly's career on Instagram, sharing a link to his new single, "Concert for Aliens," on her story. The actress previously appeared in Kelly's music video for his song "My Bloody Valentine."
The couple, who met on the set of their upcoming movie, Midnight in the Switchgrass, went Instagram official last month when Kelly shared a pic of himself and Fox.
"Waited for eternity to find you again ... ??❤️?," he captioned the photo.
Fox and Kelly's relationship comes on the heels of her May split from Brian Austin Green, whom she was married to for nearly a decade. While Fox has moved on with Kelly, Green has been spotted with Courtney Stodden and Tina Louise.
During a recent appearance on Dax Holt and Adam Glyn's Hollywood Raw podcast, Green -- who shares Noah, 7, Bodhi, 6, and Journey, 3, with Fox -- opened up about dating after divorce.
"There's no rule book to it. There's no way of doing it right or doing it wrong. We are learning as we go," he said. "The important thing for us is just communicating as much as we can."
"I wish Megan the absolute best in everything," Green added. "I want her to be completely happy, for her, for the kids. That's super important."