A triple-mode wideband bandpass filter (BPF) is proposed. It consists of a single rectangular waveguide cavity with two small metal rods to be inserted into the cavity from input/output feeding ports. As such, one TM mode, one TM mode, and one TM mode can be simultaneously excited in this cavity to form up a wide frequency passband. The fractional bandwidth is 58% at center frequency of 5.5 GHz with low insertion loss (<0.5 dB). Furthermore, this design approach is good for miniaturization compared with traditional single-mode filter. For validation, a proposed filter is fabricated using silver-plated aluminum metal cavity. Both theoretical and measured results are presented with good agreement.