Azman Porloh, a 29-year-old suspect in several terrorist attacks in Songkhla province, is facing more than five years' imprisonment for the illegal possession and accumulation of weapons.
It is the first case he is facing that has gone to trial.
Na Thawi Provincial Court in Songkhla on Thursday sentenced Azman to four years and 16 months in jail on charges of unlawful military and arms accumulation as well as unauthorised possession of arms.
He was arrested with four associates on Sept 17 at a checkpoint on Highway 42 linking Pattani and Narathiwat.
During the arrest, police discovered a flag with the logo of the BRN separatist movement and evidence suggesting planned acts of violence in his vehicle.
Azman is from Thepha district in Songkhla. His criminal background includes 18 outstanding warrants, 13 related to security and terrorism charges and five to vehicle theft.
His alleged terrorist attacks include multiple attacks on public officials, bombings and infrastructure sabotage.
He is also linked to a 2019 bomb attack on a security patrol in Songkhla's Saba Yoi district, which injured six officers.
In December 2022, he allegedly planted a bomb on the Hat Yai-Padang Besar railway line, causing a cargo train derailment, followed by an ambush on railway staff, resulting in three fatalities and four injuries.
The court's latest judgement did not address these violent incidents but focused on charges related to possessing and storing weapons without authorisation.