50 Cent: Arrested ... You Can't Say 'Motherf*****' in the Caribbean

11:00 AM PT -- 50's rep says the rapper was only booked to host, not perform. The rep says the concert was a great success and "for future trips to St. Kitts he will leave the motherf****** in the United States."

We're told he's already paid his fine and has gotten outta Dodge for the US of A.

50 Cent was arrested in St. Kitts in the Caribbean Saturday night ... for using a word that's apparently illegal there ... 'motherf*****."

Sources tell TMZ, 50 had been warned not to use profanity during his performance in front of 40,000 people. We're told the DJ didn't have a clean version of his set, and 50 didn't pull the mic away from his mouth when he roared, "motherf*****" ... and that was enough to get him arrested.

Cops showed up after the performance and took 50 to the station, where he was booked for using profanity in public. It's a minor offense, but he has to stick around until Monday when he'll go to court and pay a fine.