The 17-year-old posted a mirror selfie Sunday of her black two-piece with cutouts along the sides.
"Kourt & Ky swim session," she captioned the Instagram, referring to older sister Kourtney Kardashian.
Kylie has quite the collection of black bikinis – even Kim Kardashian West is envious! Last March, the mom to North West flaunted her post-baby body in a suit she stole from Kylie during a family trip to Miami.
She's also wearing a black bikini in the promotional poster for her 18th birthday bash at a night club in Montreal, Canada.
As for dad Caitlyn, she admitted in Sunday night's episode of I Am Cait that she's in "no rush" to pose in a swimsuit for the first time since transitioning from male to female.
"I just don't know if I'm ready to expose myself like that," she explained.