Caitlyn Jenner ain't the only one in the fam rockin' gold medals now ... 'cause Kendall Jenner is sporting her boo's new hardware during a lake vacation with Devin Booker!!
Of course, the 24-year-old just returned to America after Team USA dominated at the Tokyo Olympics earlier this month ... securing gold in men's basketball.
Book's back stateside ... and he didn't waste anytime meeting up with his supermodel girlfriend for some lakeside R&R after weeks away hooping.
Devin -- who also brought along his pooch, Raven -- posted video of the superstar couple on a motorboat .