Somsak Kaosuwan, director of the National Disaster Warning Centre said the epicentre of the quake, which hit the province at 12.49pm, was in Muang district.
Many people felt the tremor and ran out of buildings in panic. The hit was followed by several minor aftershocks.
A quake monitoring official said that the tremor took place on the active Ranong fault. Tremors had previously been caused by this fault in 1990 and again in 2009.