The 25 year old uber-popular fitness model (she has almost 13 million Instagram followers!!) put on a pink bikini and hit the beach in Cabo San Lucas ... and braved the waters for an impromptu photo shoot.

We're told Jen spent the past couple weeks on vacation in Mexico with her mom and sister ... but, as you can see, she took a break from her break to hit the beach for some snaps.

Selter's known to take her booty on tour around the world -- from Greece to Mexico to Miami -- and this batch is sure to heat you up during the polar vortex.

Enjoy!!!