A porn star shooting an underwater ad for a sex cam company was attacked by a shark and badly bitten.

Molly Cavalli dropped down under water into a shark cage wearing a hot white swimsuit and her presence apparently enticed a 10-foot lemon shark.

Check out the vid ... Molly panics when the shark approaches, and next thing you know she's screaming as she clutches her bloody foot.

The crew pulled her up to safety, but the shark had done some serious damage ... it took 20 stitches to close the wound.