"My Valentine. Happy Valentine's Day!" the "CODA" actor, 33, wrote alongside a photo of the "DWTS" pro, also 33, embracing him by the water.
Stewart also shared a glimpse of her and Durant playfully goofing around in an Instagram video set to Montell Fish's song "Fall in Love with You."
The couple's "DWTS" family expressed their excitement at the news this week by swarming the comment sections of both posts.
Fellow "DWTS" pro Witney Carson commented on Stewart's post, writing, "Is this a soft launch??? love you both!"
Pro Emma Slater, meanwhile, commented on Durant's post, writing, "Ahhhhh there it is!!!! I love this SO MUCH!!!"
Pasha Pashkov, also a professional on "DWTS," commented on both posts, writing, "Happy Valentine’s Day you lovebirds!!!"