A wideband fifth-order bandpass filter (BPF) is proposed and designed using the U-slotted substrate integrated waveguide (SIW) cavities. The U-shape slots, etched on the SIW cavity, are used to form up a new multiple-mode resonator (MMR) in order to achieve a wide passband of operation while keeping the overall size of the filter circuit to be much compact. The proposed fifth-order SIW filter is 2.5 times in size reduction compared with the traditional fifth-order SIW filters using the single-mode SIW cavities. In order to prove the validity, one wideband SIW BPF on a single layer printed circuit board (PCB) is designed and experimentally examined. The measured results show that the filter achieves an insertion loss of 1.1 dB at 8.5 GHz, return loss of higher than 11 dB and 3 dB fractional bandwidth of 42%. The measured results are in good agreement with the simulated results. © 2001-2012 IEEE.