Carrier mobility in solids is generally limited by electron-impurity or electron-phonon scattering, depending on the most frequently occurring event. Three-body collisions between carriers and both phonons and impurities are rare; they are denoted supercollisions. Elusive in electronic transport they should emerge in relaxation processes as they allow for larger energy transfers. This is the case in undoped graphene, where the small Fermi surface drastically restricts the allowed phonon energy in ordinary collisions. Using electrical heating and sensitive noise thermometry we report on supercollision cooling in diffusive monolayer graphene. At low carrier density and high phonon temperature the Joule power P obeys a P Te3 law as a function of electronic temperature Te. It overrules the linear law expected for ordinary collisions which has recently been observed in resistivity measurements. The cubic law is characteristic of supercollisions and departs from the Te4 dependence recently reported for doped graphene below the Bloch-Grüneisen temperature. These supercollisions are important for applications of graphene in bolometry and photo-detection. PUBLICATION ABSTRACT