Mantle convection and plate tectonics are driven by the heat flow from Earth's core to the surface. The radiative contribution to heat transport is usually assumed to be negligible. Here, we report the near-infrared and optical absorption spectra of silicate perovskite, the main constituent of the lower mantle, to 125 gigapascals. Silicate perovskite remains quite transparent up to the pressures at the core-mantle boundary. Estimates of radiative thermal conductivity derived from these spectra approach 10 watts meter−1 kelvin−1 at lowermost mantle conditions, implying that heat conduction is dominated by radiation. However, the increase in radiative conductivity with temperature (T) is less pronounced than expected from a T3 dependency.