AbstractRecent high‐velocity friction experiments on room temperature rocks reveal a dramatic weakening caused by extreme frictional heating and thermal degradation of the strength of microscopic asperity contacts on a slip surface, i.e., “flash” heating. Here we explore the effects of elevated ambient fault temperature, up to that at the base of the Earth's seismogenic zone, on the flash‐heating behavior of quartzite, Westerly granite, and India gabbro. The experiments demonstrate that elevated ambient fault temperature causes 1) an increase in the characteristic weakening velocity for flash heating, and 2) an increase in the friction coefficient at a given velocity above this characteristic weakening velocity. These unexpected results are reconciled with flash‐heating theory when temperature‐specific values of various material parameters are incorporated in existing constitutive descriptions of flash heating. Our results suggest that constitutive equations for flash heating can be extrapolated with confidence to the elevated ambient temperatures of the seismogenic zone.