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Chris Brown has a message for Rihanna ... and any other guy who might want a piece of her ... back off, because "THAT IS MY P**SY!"

Chris was partying at Emerson Theatre in Hollywood early Thursday AM when he grabbed the mic from the DJ and went off on how to talk to your woman -- and the bizarre rant was all caught on tape.

Chris said, "You GOTTA say that one thing to her ... Don't make me have to tell you again, that's my p***y baby! ... so you better not give it away!"

Chris rambled on ... "So every person in this motherf**king building, if you got a bad bitch you better say that s**t to her, or she might f**k another n***a."

Another girl tip ... don't beat your woman.