A new approach for the design of a balanced bandpass filter (BPF) is presented in this paper. Stemming from the mode degeneration properties of a square patch resonator, a complete set of resonant modes in a right-angled isosceles triangular patch resonator (RAITPR) is thoroughly investigated so as to construct a modified RAITPR with one electric wall and two magnetic walls along its three sides for filter design. This proposed resonator can support the half-TM10-mode of the RAITPR, and thus, it is named as the half-mode RAITPR here. By properly adopting the relationship between one RAITPR and two coupled half-mode RAITPRs, a second-order coupling topology is presented. Its working principle is then explained by using the field distribution of the desired resonant modes involved. A few high-order balanced filters are in final implemented and fabricated to evidently demonstrate their good differential-mode filtering selectivity and high common-mode suppression. Good agreement is achieved between the simulated and measured results for all the designed three balanced BPFs. To further improve the concerned out-of-band rejection performance, the feeding scheme of the modified structure has been discussed.