Justin Bieber -- Topless Mexican Pics with Selena Gomez

Justin Bieber was smothered by his smokin' hot adult girlfriend Selena Gomez in Mexico yesterday ... and was so impressed by her incredible physique ... he snapped a few pics to capture the moment.

The couple is south of the border for a friend's wedding ... but yesterday, they spent the day taking in some sun at the hotel pool ... because, why not?