This article presents a high-efficiency circularly polarized slotted waveguide leaky wave antenna at the W band. A linearly polarized beam-steering antenna is designed by creating an array of longitudinal slots on the WR-10 waveguide and to achieve circular polarization, a single-layer polarization converter has been used. A new technique has been proposed to enhance the overall efficiency of the antenna. The wall thickness of the waveguide containing the radiating slots is reduced to increase the overall radiation efficiency of the provided antenna. The gain of the prototype antenna is above 20 dBi in the intended operating band from 92 GHz to 98 GHz. Furthermore, the antenna provides an average half-power beamwidth (HPBW) of 3.2° in the elevation plane and a cross-polarization below -18 dB in the main beam direction. Attributed to robust structure, reliable fabrication, and simple antenna configuration as well as its superior performance, the proposed structure is a strong candidate for various W band applications.