11-year-old Winter attended Post's Tampa Bay concert Tuesday night, and at the end of the show, he handed over his personal blue guitar ... the same one he jams out with. Oh, and we should mention, he signed it too.

Winter's mother, Jenn, tells TMZ ... her kiddo's a massive Post fan, as is her other daughter, Brooklyn. Tampa's show marks the 5th PM show the family has attended. At past shows, Jenn says they've seen Post give away smashed guitars to fans, but nothing like this.

We're told Winter doesn't know how to play the guitar, but the rare gift might be enough to get her into learning. In handing it over, Post made the young girl promise to treat the instrument with respect, and Jenn says that's exactly what they'll do.

Winter plans to put it in a case to display on a wall in her room and will never sell it.

Don't worry, Brooklyn didn't walk away empty-handed -- she ended up with Post's sweaty shirt and socks. Not a bad consolation prize!