A triple-mode wideband bandpass filter (BPF) is proposed in this paper. Three resonant modes, namely, TM mode and A pair of TE degenerated modes, have been excited in single rectangular waveguide cavity. Meanwhile, a wide pass bandwidth have been realized and the fractional frequency bandwidth of about 29% is achieved at the central frequency of 3.1 GHz. Finally, a filter sample is fabricated and tested. The measured results are well matched with the simulated one.