A new balanced-to-balanced f ltering power divider based on the square patch resonator is proposed in this paper. Originating from the f eld distributions of degenerate modes on the square patch resonator, a prototype of the balancedto- balanced power divider is successfully developed with only one square patch resonator. Its working principle can be fully explained by the f eld distribution of the relevant resonant modes. Afterwards, to achieve the sharp skirt selectivity outside the passband, a compact second-order circuit is constructed based on the original prototype. With this new structure, both good bandpass filt ring response and power division characteristics are well obtained. Moreover, to suppress the differential-mode (DM) harmonic response in the upper stopband, an improved structure is built up by modifying the feeding scheme. Ultimately, to validate the design concept, both the two types of secondorder balanced-to-balanced f ltering power dividers operating at 1.8 GHz are designed and fabricated. The measured results of the two fabricated circuits well agree with the simulated ones, manifest equal power division, high performances in terms of DM response, in-band common-mode (CM) suppression, port-to-port isolation, and expected harmonic suppression.