The Y-matrix of a three-port dual-mode ring resonator for a balun-bandpass filter (BPF) application is derived. Using Y parameters, it is verified that the structure theoretically satisfies the general balun conditions. The admittance slope parameters of the dual-mode ring resonator with one terminated port are derived so that one can design balun-BPFs using the conventional standard BPF design procedure. The proposed equations are validated by designing a balun-BPF. Inverters are then substituted with low-pass filter unit cells of λ/4 electrical length to suppress harmonics from the second to the fourth. The experimental measurements show good agreement with theoretical and electromagnetic-simulated predictions and suppressed harmonics below λ30λdB up to fourth-order harmonics.