The rapper was performing in Biloxi, Mississippi Tuesday night when the woman somehow got on stage and started dancing next to Afroman -- and then suddenly, without warning ... he turned and landed a right-handed haymaker to her face.
He obliterated the woman ... who went down hard -- but Afroman kept on performing. Witnesses say she was bleeding and crying, but somehow she managed to get up on her own.
Cops eventually stopped the show, escorted him off stage, and arrested him outside the venue. He was booked for assault.
Afroman's rep says the rapper had no idea if the fan was a man or a woman -- he just reacted to someone being on stage. The rep claims there was a lack of security at the venue to keep fans off the stage.
The rep added this is totally out of character for Afroman ... who was released late Tuesday after paying a $330 bond.