We have measured the temperature rise in nanoporous silicon under strong illumination. A green laser beam was focused with a microscope objective on porous silicon films with porosities between 55% and 80%. The Raman spectrum was measured for power densities between 0.8 and 65 KW/cm$^2$. We obtained the temperature of the illuminated area from the shift of the phonon frecuency when phonon confinement effects are removed. The temperature depends linearly on power density. For a given power density, the temperature increases with porosity for porosities below 60% as the thermal conductivity decreases. Beyond that point, the temperature decreases because the reduction of light absorption dominates. The maximum temperature reached was 400 $^∘$C for a 60% porosity sample and for a power density of 65 KW/cm$^2$.