In order to clarify the characteristics of long-term changes of water temperature in the inner part of the Ariake Sea due to climate change, time series data of water temperature and atmospheric temperature observed from 1973 to 2023 were analyzed. Representative values for each year were calculated by applying an appropriate periodic function to their temperature data. Changes of representative values indicated that the water temperature in the inner part of the Ariake Sea has been increasing in the long term. In addition, there will be a change in the influence from river water since the rate of increase in water temperature are different between coastal zone areas and central parts of the sea. Therefore, the long-term changes of a river water temperature and its river discharge were examined. The results showed that increases of the atmospheric and the river water temperature due to the global warming have led to further increase in coastal water temperatures, and the spatial water temperature distribution in the inner part of the sea has recently become more uniform.