12:57 PM PT -- Nia tells TMZ she's NOT calling cops to report Soulja. She says she doesn't have a BF, and the person he threatened is just an Internet troll trying to piss off the rapper.
A rep for Soulja claims the gun was a fake, and adds he's already reached out to Nia to apologize.
Soulja Boy is making disturbing threats against the guy he thinks took his place ... with his ex-girlfriend.
The "Crank That" rapper made the vicious threat Friday in a video post ... waving and pointing what appears to be a Glock with an extended magazine.
It's unclear why the threat's being directed at Nia Riley and her BF. Not the smartest move -- especially considering Soulja's on probation for a weapons charge.
We've reached out to Soulja, so far no word back.