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.

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.