The woman claiming Soulja Boy fathered her kid has some hard evidence he's the daddy ... including pictures of her straddling Soulja.
Diamond Shantaniece Mackey filed legal docs claiming Soulja Boy has a son -- 5-year-old Portlon.
Turns out Diamond may have struck gold with a picture taken in 2007 in a hotel room where she claims she took a pregnancy test ... which came back positive.
During her pregnancy, Soulja became a huge star with the hit, "Crank That." After Portlon was born ... Diamond says Soulja took a home DNA test and texted her a picture of a 0% sign and then changed his phone number.
Diamond says she always believed Soulja to be the father, she even told her case worker shortly after Portlon was born.