Police Colonel Sathit Polpinit said that four gunmen opened fire at a group of villagers giving alms to a Buddhist monk in Mae Lan district in Pattani province.
He said a 68-year-old monk, one woman and her 13-year-old son, and another woman were killed in the attack.
Sathit said six others, including a police officer in a squad protecting the monk, were wounded.
Police said they were looking for the gunmen who fled on two motorcycles.
More than 5,000 people have been killed in Thailand's Muslim-dominated southern border provinces since an Islamic insurgency erupted in 2004.
