Jay Z had a love child 21 years ago and is now blocking the guy's efforts to prove he's the son of the rap mogul ... according to legal docs.
The guy's name is Rymir Satterthwaite and he claims his mother hooked up with Jay back in the early '90s -- before he was world famous, and long before Beyonce was in the picture.
Rymir and Lillie Coley -- who calls herself his legal guardian -- say in a civil suit ... Jay Z submitted fraudulent information to the court ... to intentionally torpedo Rymir's paternity claim.
It's unclear what that fraudulent info is -- but there are reports that Jay is refusing to take a paternity test.
Rymir happens to be an aspiring rapper ... who's posted vids of himself rhyming.
The civil lawsuit was handwritten by Coley ... who claims Jay's actions have caused Rymir financial and emotional damage. We've reached out to Jay Z's attorney, but so far no word back.