Nikki Sixx’s GF — Preps for Motley Crue STRIPTEASE

Nikki Sixx is 53 … his girlfriend is about to turn 27 … and she plans to celebrate by stripping down to her undies for the rocker — and 15,000 of his closest friends.

The birthday girl is Courtney Bingham … and she”s hot. Real hot. We have pics to prove it.

Anyway, we”ve learned … she”s planning a special burlesque performance to “Girls, Girls, Girls” … and she plans to unveil the hotness on her birthday (9/5) during a Motley Crue show in Michigan.

FYI — Court has been dating Sixx since 2010 — after his rocky romance with Kat Von D bit the dust — and they have been inseparable ever since.

Now we see why.