Selena Gomez -- Body Banging and Bikini Clinging ... Pinky Swear

Selena Gomez didn't pull any strings in Puerto Vallarta ... dammit.

Biebs' former squeeze was showing off the goods during her Mexican vacay ... sporting a double-stringed bikini bottom thingy (we know fashion).

There's not a ton of material to her ensemble, but what there is -- is definitely working overtime to cover the 22-year-old's assets. Things are getting thick down in Mexico.