Knowledge of the thermal conductivity and water content relationship is required for studying heat and water transfer processes in partially saturated soils. None of the available models is able to give satisfactory results for the entire range of soil water content. In this study, we propose a new model that describes the relationship between soil thermal conductivity and its volumetric water content. The input parameters for the model are soil thermal conductivities at dry and saturated states and saturated soil water content, which can be determined by the thermo-TDR technique. Laboratory tests were conducted on soil columns of different textures and bulk densities. The results indicated that in the entire water content range, the RMSE (room-mean-square-error) of the new model was less than that of the Kerstan model (1949), de Vries model (1963), and Johansen model (1975). The model also provides a valuable tool for estimating soil hydroscopic water content and capillary water content.