Pam Anderson ain't exactly shy about public nudity ... so when the opportunity came up for the 46-year-old to go top down at a topless beach in France ... out came the gals.

Pam was with her BF Rick Salomon -- and the two were spotted romping around the beaches in Biarritz, France yesterday like two topless teenagers.

Fun Fact -- Biarritz is the birthplace of famed chemist Ernest Fourneau -- who played a key role in the development of chemotherapy as a way to combat cancer.

... and now you know.