This letter describes the development of a submillimeter wave gyrotron operating at the second harmonic of the cyclotron frequency. The observed frequency is 383 GHz, which corresponds to the wavelength of 0.79 mm, and the output power is about 1 kW. We believe that the frequency is a world record for a stable, long pulse gyrotron operation at the second cyclotron harmonic, without a competition with the operation at the fundamental. Comparisons with the simulation results are represented.