Previous researchers have shown that thermal conductivity and electrical resistivity are related to the water content and void ratio of soil. The objective of this study is to present a theoretical relationship between these two physical parameters. A de Vries equation and Archie’s law are applied to develop a new theoretical equation that relates thermal conductivity to the electrical resistivity of soil. The DRE-2C thermal conductivity tester, which uses a transient plane-source method, is used to measure the thermal conductivity. In addition, the DDC-8 resistivity meter is used to measure the electrical resistivity. Experiments on the thermal conductivity and electrical resistivity of silt soil and silty clay soil with different gravimetric water contents and densities are performed. The results indicate that the theoretical equation can well explain the relationship between the thermal conductivity and electrical resistivity of silt and silty clay soils. The thermal conductivity and electrical resistivity are also found to have a linear relationship with the density of silt soil. When the gravimetric water content is less than 30%, the thermal conductivity and electrical resistivity of silty clay soil increase linearly with the density. The thermal conductivity increases with the gravimetric water content to a critical threshold depending on the soil type. The silty clay samples with a water content of 20% have the largest value of thermal conductivity. The electrical resistivity of the silt and silty clay samples decreases rapidly due to the increased pore connectivity and enhanced hydration of ions in soil with the increased water content. The results of the experiments indicate that the new theoretical equation is effective for estimating the soil electrical resistivity from the soil thermal conductivity.