The climatology and linear trend of seasonal water temperature and heat budget in a semienclosed sea connected to the Kuroshio region (Seto Inland Sea: SIS) are investigated by constructing and analyzing a gridded data set of water temperature, salinity, and air-sea heat fluxes. In the SIS, winter-time water temperature and ocean heat content (OHC) showed a significant increasing trend while those in summer-time exhibited no significant change. By analyzing the heat budget between the tendency of ocean heat content OHC/t and air-sea net heat flux Q(net), the driving factor of the winter-time warming trend in the SIS was identified as autumn Q(net), while the unchanged summer-time OHC is mainly due to decreased spring-time oceanic heat transport to the SIS from the Kuroshio region. In addition, we showed that the decreased spring-time heat transport could have been induced by upwelling due to wind curl and wind speed in the shelf-slope region, heat transport by the Kuroshio south of Shikoku Island, and freshwater input to the SIS from rivers. The importance of both oceanic and atmospheric forcing mechanisms for explaining variation in the water temperature in coastal areas is demonstrated.