Telfair -- a 33-year-old former high school phenom who spent 12 seasons in the NBA -- was busted back in June 2017 in Brooklyn .... after cops found several weapons during a traffic stop.

Officials say Telfair -- who played with the Trail Blazers, Celtics, Clippers and Timberwolves, to name a few -- had 3 pistols and what appeared to be a semi-automatic rifle.

Cops say he was also carrying a ton of ammo and a bulletproof vest ... as well as some weed.

Telfair pled not guilty and fought the charges ... but was found guilty by a jury in NYC on Wednesday of felony criminal possession of a weapon, according to our law enforcement sources.