A novel λ/4 microstrip stepped-impedance resonator with interdigital coupling elements is presented in this paper. By combining the merits of these two structures, strong coupling between adjacent resonators for wideband bandpass filters can be easily obtained and the size of the filter can be largely reduced. More importantly, only a single-sided fabrication process is required in the implementation of the proposed filter. Two four-pole bandpass filters with 0.1 dB equal-ripple bandwidths of 38% and 55%, respectively, are designed and fabricated. Both filters have the advantages of compactness, wide bandwidth, and ease of fabrication.