A bomb hidden in a parked car exploded in a business district of Yala city, about 100 meters (110 yards) from a food market at 8:10 a.m. (0110 GMT), killing the ranger instantly. Four rangers were wounded.
Nineteen civilians, most of them Buddhists, were also wounded, police said.
Yala is one of the three Muslim-dominated provinces bordering Malaysia where more than 4,500 people, both Muslims and Buddhists, have been killed in a low-level insurgency since 2004.
Analysts believe the unrest, for which no group has claimed responsibility, is an ethno-nationalist campaign by ethnic Malay Muslims who say their identity, language and culture is neither respected nor fully understood by the Buddhist-dominated Thai state.
Despite the deployment of 60,000 members of the security forces and police in the rubber-rich region, authorities have made little progress in ending the revolt.
