Rapper Lil B -- James Harden Needs to Stop Doing My Dance ... OR ELSE!!!

The biggest obstacle standing between James Harden and an NBA championship is not Steph Curry ... it's Lil B, who is threatening to put a curse on The Beard ... unless he cops to his demands.

Lil B is pissed because he thinks Harden keeps jacking his "cooking dance" when he celebrates draining a big shot. Harden has been doing it all season, but Lil B says he's watching the Western Conference Finals -- and if Harden does it again ... it's gonna be curses.

Lil B tells TMZ Sports, "If Harden doesn't tell what he is doing, which is the Lil B cooking dance, he will be cursed. He needs to stop stealing Lil B swag without showing love."

Lil B is a reasonable man -- he's giving Harden until Game 3 to give him credit, but after that he says the curse goes into effect.

Not a believer? Remember, Lil B cursed Kevin Durant earlier this year, and uhh ...he's somewhere chillin' on a couch right now.

Stir at your own risk, Harden.