A microwave inverted-cupstron tube radiating at cyclotron frequencies is designed and analyzed. This device utilizes the negative mass instability for the resonant interaction between an axis encircling electron layer and the modes of a cylindrical waveguide. The outer wall is a conducting circular waveguide and the inner wall has a slotted structure similar to the outer walls of the anode blocks of a magnetron. Similar to a noninverted cupstron tube the E layer configuration maximizes the finite Larmour radius effect for the harmonic cyclotron resonance interaction. The slotted inner boundary introduces a periodic fringe field for enhancing the harmonic resonant interaction. A parametric study of the operational characteristics of the device was performed for determining the optimum design parameters and the beam energy.