A compact branch-line coupler with harmonic suppression is presented. By analyzing the characteristics of stub-loaded transmission lines, it is found that a stub-loaded transmission line with a smaller size can be used to equivalently substitute the quarter-wavelength transmission line in the traditional branch-line coupler. As a result, the size of the branch-line coupler is significantly reduced. Furthermore, due to the transmission zero introduced by the open stubs, the harmonic response of the coupler is suppressed. To validate the idea, the equivalent circuit model was developed, analytical closed-form equations were derived, and circuit simulation results verified the design principles.