He was identified as Jehusman Hayikaseng, 46, a defence volunteer of Muang Pattani district.
Pol Lt Chaiyan Bandith, duty officer at Nong Chik police station, said the shooting occurred about 7pm on Don Rak-Tha Dan road at Ban Don Rak in Tambon Don Rak.
According to witnesses, Jehusman was travelling along the road when a gunman riding pillion on another motorcycle fired at him with a handgun. The assailants then fled.
Jehusman took two bullets in his chest and died on the spot.
Police blamed separatist militants.
Almost 6,000 people have been killed in the southern violence since Jan 4, 2004 which marked the onset of separatist unrest, according to the Internal Security Operations Command Region 4 Forward Command.
A total of 5,926 people died as a result of the sectarian violence in Yala, Pattani, Narathiwat as well as Chana, Thepha, Saba Yoi and Na Thawi districts of Songkhla from Jan 4, 2004 to Dec 31, 2013 last year,
Of the dead, 3,461 were Muslims and 2,431 were Buddhists. Some 10,593 people have been injured.