A triple-passband filter is proposed by exploiting the first three resonant modes in a single rectangular cavity: TE ₁₀₁ , TE ₀₁₁ , and TM ₁₁₀ modes. Triple-passband frequency response is realized by generating one transmission zero (TZ) between any of two adjacent modes so that each mode contributes one passband. Beside TZs between modes, the three resonant modes are excited by properly placing two coupling apertures at input/output ports of the cavity. When cascading two single cavities, three pairs of slots are used to independently control the coupling between two respective modes of different cavities. As a result, not only two TZs between different passbands but also a pair of additional TZs below the first passband and above the third passband could be realized, respectively, thereby attaining a high selectivity of a triple-passband filter. Finally, a dual-cavity filter with triple-passband and four out-of-band TZs is fabricated and tested. The measured results have verified the validity of the proposed design well.