This work investigates the temperature-dependent thermal conductivity of the heat-treatable steels C45, 40CrMnMo7, and X42Cr13 in a high-tempered condition. The results reveal that the temperature-dependent evolution of the thermal conductivity is strongly influenced by alloying composition. Furthermore, not only the thermal diffusivity but also the specific isobaric heat capacity has a major impact on the resulting thermal conductivity at higher temperatures. The results are discussed with respect to the resulting microstructures and under consideration of Calphad calculations. The results are relevant for the thermal design of tools, particularly those used for high-pressure die casting.