A high-power sheet beam electron gun for a quasi-optical gyrotron has been developed. The gun produces two beams with rectangular cross sections, with one beam segmented into 11 beamlets. These beamlets are spaced so as to be in line with the maxima of the RF field profile in the resonator of a 120-GHz quasi-optical gyrotron. The maximum current in the segmented beam is nominally 34 A, while the that for the unsegmented beam is 68 A. The gun has been tested, and has operated essentially as predicted by 2D and 3D theory. In particular, the overall beam shape is largely undistorted, and the beamlets are, for most cases, well aligned.