In this Letter, a high‐frequency, second‐harmonic gyrotron using a 12 T superconducting magnet is described. It has achieved a maximum frequency of 636 GHz (corresponding to a wavelength of 472 &mgr;m). Operating at both the fundamental and the second harmonic of the electron cyclotron frequency enables the gyrotron to act as a high power (several 100 W), step tunable, millimeter to submillimeter wave source in the wide frequency range from 150 to 630 GHz.