Numerical simulations of three-dimensional convection with temperature-dependent viscosity and viscous heating at realistic Rayleigh numbers for Earth’s mantle reveal that, in the strongly time-dependent regime, very intense localized heating takes place along the top portion of descending cold sheets and also at locations where the ascending plume heads impinge at the surface. For a viscosity contrast of 100, these localized heat sources exceed the internal heating due to the radioactive decay of chondritic materials by more than an order of magnitude. The horizontally averaged viscous dissipation is concentrated in the top of the convecting layer and has a magnitude comparable with that of radioactive heating.