Jen Selter -- My Mom's Hot Too ... Check Out This Body!!

Apparently, the apple bottom doesn't fall far from the tree.

Here's 21-year-old butt selfie goddess Jen Selter (right) posing with her 47-year-old mom Jill Selter (left) ... showing off their incredible mother-daughter physiques in tiny white bikinis.

Jen -- whose ass has a 5 million person following on Instagram -- says her mom hits the gym 4 to 5 times a week ... and clearly, it's paying off.

FYI -- Jen is also her mother's personal trainer ... so ... good work!