Police said four assailants on two motorbikes opened fire killing Yacob Raimani, imam of the Pattani central mosque, while he and his wife were buying food for their meal after fasting.
Imam Yacob Raimani, 49, an outstanding religious leader in Pattani, had greatly contributed to the development of Muslim religion, education and human resources.
He was attacked once before, two years ago, but managed to escape unharmed at that time.
The shooting took place during the holy month of Ramadan. Thai and Muslim insurgent negotiators have said they would attempt to halt all violence in insurgency-plagued provinces throughout the Islamic fasting month.