Young Thug's brother, Unfoonk, is a return inmate at Fulton County jail -- law enforcement sources tell TMZ Hip Hop he's been arrested and booked for firearm possession by a convicted felon.
Unfoonk joined Gunna last December in entering a plea in the high-profile YSL RICO trial ... but his latest run-in with the law could lead to a violation of his probation terms.
Cops in Atlanta say Unfoonk (real name Quantavious Grier) was pulled over in his neighborhood around 4 PM Thursday ... and during the stop, officers claimed to have smelled burnt marijuana inside the black Mercedes sedan.
We're told officers also say there was excessive tint on Unfoonk's windows, and the YSL rapper wasn't wearing his seatbelt.
They called in a police K9 to help with the narcotics search of the vehicle, and during that search ... we're told officers located a handgun in the driver's side door panel. As the owner of the car, Unfoonk was arrested for gun possession.
According to the docs, he was also dinged for criminal street gang activity -- and the much smaller infractions of improper window tint and a safety belt violation.
YSL's RICO trial remains ongoing and has been the center of nonstop hijinks ... this latest chapter included.