Heat transport and dissipation at the nanoscale severely limit the scaling of high-performance electronic devices and circuits(1). Metallic atomic junctions serve as model systems to probe electrical and thermal transport down to the atomic level as well as quantum effects that occur in one-dimensional (1D) systems(2). Whereas charge transport in atomic junctions has been studied intensively in the past two decades(2-5), heat transport remains poorly characterized because it requires the combination of a high sensitivity to small heat fluxes and the formation of stable atomic contacts. Here we report heat-transfer measurements through atomic junctions and analyse the thermal conductance of single-atom gold contacts at room temperature. Simultaneous measurements of charge and heat transport reveal the proportionality of electrical and thermal conductance, quantized with the respective conductance quanta(6). This constitutes a verification of the Wiedemann-Franz law at the atomic scale(7).