A novel calorimeter for millimeter wave radiation based on the photoacoustic effect has been investigated. Single pulses from a 3‐&mgr;s, 1‐MW gyrotron operating between 135 and 150 GHz were measured. The joulemeter has both a slow time response (<500 Hz), due to gas expansion, and a fast time response (≳500 Hz), due to acoustic modes of the resonator and windows. The slow time response, which was more suitable for power and energy measurements, was compared with a calibrated thermopile detector and was found to be linear over the range of 0.15–3 J, with an error of less than ±5% near 3 J. The calorimeter was also tested with modes having different power distributions in space; the response was found to be independent of spatial power distribution. The joulemeter is a simple, rugged device that can be used in transmission or absorption mode. Use in transmission mode is attractive for basic research on high‐power microwave sources.