Rainfall is an important component of water cycle, but excessive rainwater can lead to flooding which impacts many sectors. There is an urgent need to investigate the extreme rainfall event in southern Thailand since that region has high risk of flooding. This research aims to investigate the extreme rainfall trend in the southern Thailand. Daily rainfall data from 17 meteorological stations in the area were used to calculate total precipitation (PRCPTOT), maximum 1-day precipitation (Rx1day), and number of days with ≥20 mm rainfall (R20mm) in each season from 1981–2020. The trends of PRCPTOT, Rx1day, and R20mm were investigated by Mann-Kendall test at 0.05 significance level and Sen’s Slope estimator. The results reveal that the PRCPTOT and Rx1day significantly increased in both coasts of southern Thailand during summer while R20mm significantly increased in the middle part of the eastern coast. In winter, PRCPTOT and R20mm significantly increased in the middle part but significantly decreased in the northern part of the eastern coast. However, during the rainy season, the trends of these indices were not obvious. These findings imply that southern Thailand may have more extreme rainfall especially in summer (for both coasts) and over the middle part of the eastern coast (in summer and winter).