TMZ broke the story ... 21-year-old Soulja and 4 other men were arrested in Temple, GA early Tuesday morning ... after cops pulled over their Escalade because one of the lights on the vehicle was out.

Sources tell us ... when cops approached the car, they detected a "strong odor" of marijuana ... and searched the SUV.

We're told cops found a briefcase during the search, which they believe belongs to Soulja Boy ... and inside, officers claim they found a "substantial amount" of weed and a firearm.

Law enforcement sources tell us ... all 5 men were hauled to a nearby jail and booked for marijuana possession and a gun charge.