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.