The subsurface temperature is of major interest for assessment of the petroleum prospectivity of a sedimentary basin. A key parameter in modeling the thermal structure of the subsurface is thermal conductivity. The information about lateral variation of this parameter is usually sparse. We propose a simple linear relationship between thermal conductivity and seismic velocity for clastic sedimentary rocks. The coefficient of proportionality depends on the clay fraction. This implies that thermal conductivity and seismic velocity have a similar sensitivity to porosity. Both thermal conductivity and seismic velocity increase with decreasing porosity. For a given porosity, quartz-rich rocks have higher thermal conductivity than clay-rich rocks, which is in agreement with experimental observations. Whenever seismic velocity models are available from seismic surveying, the proposed thermal conductivity-velocity relationship can be used to obtain laterally varying 2D and 3D thermal conductivity models for use in basin modeling.