In this letter we analyze an induced resonance electron cyclotron (IREC) quasi‐optical maser configuration, which has the unique features of being highly efficient and, with the proper choice for the index of refraction, relatively insensitive to the electron beam’s energy spread. The radiation mechanism can, however, be sensitive to the beam’s pitch angle spread. A system of nonlinear coupled orbit equations, describing the dynamics of electrons in a spatially varying magnetic field, and the electromagnetic field of a steady‐state open resonator configuration are derived and analyzed. Furthermore, we show that by appropriately tapering the magnetic field an induced resonance condition can be achieved which results in high interaction efficiencies. It is anticipated that an efficient, high power millimeter, submillimeter, and infrared radiation source can be realized with the IREC quasi‐optical maser configuration.