Joanna Krupa -- I'M NOT A HOOKER!!!!

New "Real Housewives of Miami" star Joanna Krupa insists ... she's NOT a high-priced prostitute willing to bang a stranger for $10,000 -- despite reports to the contrary -- and now, she's threatening to SUE one website over the accusations.

Krupa's attorney Marty Singer has fired off a letter to Nik Richie at TheDirty.com, claiming one of Nik's recent posts -- entitled "Joanna Krupa Is a Confirmed Hooker" -- is filled with defamatory lies.

The article in question quotes a man who claims to have had sex with Krupa for $10,000 through an escort agency -- adding, "'The Real Housewives of Miami' should have done a better background check."

Now, Joanna's threatening to file a lawsuit against Nik if he doesn't pull the article, calling it "an extremely serious matter."