And not just any bikini -- it belongs to her 16-year-old sister!

Kim posted the above shots and wrote, "Quick swim before the Dash party ... Yep, stole Kylie's bikini...she's not getting it back." We think Kylie will beg to differ, especially since she posted her own selfie in the same bikini earlier in the day!

It appears as though the sexy snapshot was taken in the bathroom of the Versace Mansion, and while Kim barely squeezed her boobies into the top, we have to say, she looks pretty incredible! We're surprised Kanye's not hopping on a jet and heading over there right now...