Increased heat stress affects well-being, comfort, and economic activities across the world. It also causes a significant decrease in work performance, as well as heat-related mortality. This study aims to investigate the impacts of the projected climate change scenario under RCP8.5 on heat stress and associated work performance in Thailand during the years 2020-2029. The model evaluation shows exceptional performance in the present-day simulation (1990-1999) of temperature and relative humidity, with R-2 values ranging from 0.79 to 0.87; however, the modeled temperature and relative humidity are all underestimated when compared to observation data by -0.9 degrees C and -27%, respectively. The model results show that the temperature change will tend to increase by 0.62 degrees C per decade in the future. This could lead to an increase in the heat index by 2.57 degrees C if the temperature increases by up to 1.5 degrees C in Thailand. The effect of climate change is predicted to increase heat stress by 0.1 degrees C to 4 degrees C and to reduce work performance in the range of 4% to >10% across Thailand during the years 2020 and 2029.