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In this letter, a postwall-slotline stepped impedance resonator (PWS-SIR) is proposed and applied to a bandpass filter (BPF) with a wide stopband. The proposed PWS-SIR-BPF comprises three U-shaped PWS-SIRs and two microstrip-slot feeding transitions. A PWS has a much lower impendence which a conventional slotline (CSL) cannot reach, so a much smaller impendence ratio of the PWS-SIR can be achieved. Consequently, a wider stopband simultaneously can be realized for the proposed filter. The designed PWS-SIR-BPF, as well as a CSL-BPF, have been fabricated, measured, and compared to verify the features of the PWS-SIR. The measured results are consistent with the simulation ones. The PWS-SIR is 7.3 mm (0.22λ0) long, 67% of 11.1 mm (0.34λ0) of the CSL resonator. The first spurious resonance frequency of the PWS-SIR-BPF is extended from 9.8 GHz (2f0) to 23 GHz (4.7f0).