Border police Sgt. Chalaem Pohpan was riding a motorcycle in Pattani province's Mayoh district at around 10:30 a.m. (03:30 GMT) when a gunman, riding on the back of another motorcycle, shot and killed him, said police Major Narat Thepcharoen.
Further details about the incident were not immediately available.
Yala, Pattani and Narathiwat provinces have recently suffered a series of violent attacks, including the torching of 21 schools and a raid on an army arsenal on 4 January in which four soldiers were killed.
At least 45 people, including Buddhist monks, hospital workers and teachers, have been killed and dozens more injured in hit-and-run attacks in southern Thailand since the start of the year.
No one has claimed responsibility for the attacks but government officials have blamed Muslim separatists with possible links to regional and international networks.
Last week police arrested nine Muslims accused of taking part in the 4 January attacks, and charged them with treason and other crimes.
The three southernmost provinces that border Malaysia are the only Muslim-dominated areas in this mostly Buddhist country. An Islamic separatist movement in the region was put down in the late 1980s, but violence resurfaced in 2001.
As part of a solution to the violence in the south, the government has proposed economic and educational development.
"Muslim youths will reap the benefits in the areas of religious teachings ... and knowledge in basic vocational courses, which can be used to support careers and release them from poverty"
Prime Minister, Thailand
Prime Minister Thaksin Shinawatra met on Monday with a dean from Egypt's Al-Azhar University, which is considering opening a branch in Thailand, and reiterated his pledge to improve educational opportunities for Thai Muslims.
"Muslim youths will reap the benefits in the areas of religious teachings ... and knowledge in basic vocational courses, which can be used to support careers and release them from poverty," a government news release quoted Thaksin as saying in Bangkok.