The rapper was charged with unlawful possession of a firearm by a felon stemming from a 2020 arrest, but the case was was dismissed this week ... according to documents obtained by TMZ.

BY's lawyer Carl A. Moore tells us his client's happy to put this case behind him after dealing with it for nearly four years ... saying he feels the District Attorney's Office did the right thing by dropping the charge -- and, justice has finally prevailed.

Youngsta -- who Moore tells us turned down a pre-trial diversion program before prosecutors ended up dropping the case -- was arrested in October 2020 during a routine traffic stop. He was riding shotgun in the car, and cops said they found a handgun tucked between his legs.

TMZ obtained video of Blac interacting with the cops -- amiably chatting with the officers ... even asking how much the van they picked him up in cost.

It was Youngsta's second gun-related arrest in as many years. In September 2019, police say Youngsta claimed a bag containing 2 semi-automatic pistols in his car belonged to him, and said he had prohibited ammunition with him.