Instagram hottie Jen Selter took her famous ASSets on a beach vacation, and it looks like she took a new bf too ... Big Time Rush singer James Maslow.

Selter and Maslow were down in Tulum, Mexico over the weekend. She's renowned on social media for her butt ... but James is pretty easy on the eyes too.

No word if they're actually dating, but check out the pics -- they seem, at the least, very friendly.

Maslow actually performed at the Maxim Spring party, so it might have started as a work trip -- but based on these pics he's mixing in plenty of pleasure too.

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