Britney Spears is letting loose on vacation ... and letting it all hang out too -- stripping down to her most see-through lingerie in her recent Instagram post.

The singer-songwriter shared a snap taken of her from behind while taking a sabbatical in Mexico ... and, HER exposed behind grabbed eyes -- basically uncovered except for a small strip of mesh fabric.

Check out the cheeky pic ... Spears bunches up her hair so nothing's covering her posterior -- walking toward a glass door with a view of a crystal clear pool.

Brit didn't write anything in the caption ... but, a picture is worth a thousand words -- even if this one is sure to leave fans speechless.

Spears has been showing off her bod quite a bit in recent days on IG ... posing in a fiery red thong just last month -- and performing a couple of her patented dance routines in various states of undress.

Britney's almost certainly posting these pics and vids because she enjoys them ... though it has the added benefit of reminding Paul Soliz exactly what he's missing. As we told you, Britney -- who has been in an on-again, off-again relationship with Paul for about a year -- dumped the guy right around Valentine's Day.

In the past, BS had been seen hanging out with Paul and his kids at various fun locales in L.A. ... but, our sources say this seems to be the final breakup -- though ya never know with this couple.