Soulja Boy -- Woman Claims Fertile Rapper Fathered Her Kid

Soulja Boy has a spitting image ... so claims a woman who says he knocked her up 6 years ago.

Diamond Shantaniece Mackey claims in legal docs -- obtained by TMZ -- there was only one other guy she slept with during the critical time period ... he was of mixed race and her kid isn't.

Diamond says her 5-year-old Portlon resembles Soulja, the rapper has visited the boy and there are witnesses who can vouch for the relationship.

Diamond says she hasn't kept it secret ... she's even told the welfare office Soulja's the daddy.

She wants a DNA test and, we're guessing, lots of child support.