A simple model to estimate the daily mean water temperature of paddy fields from observed meteorological data was developed. The model is based on a crop growth model coupled to an equilibrium water temperature model. Using this model, the water temperature of a paddy field can be estimated from only three factors namely, air temperature, solar radiation, and water vapor pressure, observed a nearby weather station. Validity of the model was examined on actual paddy fields of different style of cultivation, under warm temperate climate. There was a good agreement between observed and calculated temperature. This suggests that the water temperature of paddy fields can be estimated well using this model without any correction to adjust for the style of cultivation.