Nikki Sixx's GF Courtney Bingham -- 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.