A design of a compact wideband harmonic suppressed rat-race coupler (RRC) is presented in this paper. The present coupler is obtained by replacing each quarter wave length transmission line of a conventional double section rat-race coupler with a triple stub M-shape unit. The M-shape unit with 3 stubs is used to enhance the bandwidth, suppress the harmonics, and reduce the size of the coupler. Design guidelines are established using the lossless transmission line model. Theoretical predictions are verified by fabricating a prototype coupler. The proposed double section RRC provides harmonic suppression up to seventh of operating frequency and 62.4% size reduction with wide bandwidth, which is useful for wireless communication systems.