In this article, a new wideband diplexer for microwave frequencies is proposed. This diplexer is designed based on the combination of spoof surface plasmon polariton structures with substrate integrated waveguide (SIW) structures. In the proposed diplexer, bandpass filters were designed within a frequency range of 6–9.2 GHz (bandwidth of 40%) and 11.7–16.2 Hz (bandwidth of 29%) by creating rectangular grooves on the top and bottom of the metallic layers of the SIW. These grooves generate surface plasmon waves in the SIW structure that limit the dispersion curve to an asymptotic frequency and cause a high cut‐off frequency. The low cut‐off frequency of the filters is determined by the SIW dimensions. Moreover, the two filters were fed using a microstrip T‐junction with stepped impedance transformation. By optimizing the feed lines, the electromagnetic waves in each working band are directed to the filter of that band, and finally, the filters create a proper attenuation outside the band. A sample of the diplexer is fabricated and tested to verify the design. The simulation results are in good agreement with the experimental results. This diplexer can be a good candidate in microwave frequencies for telecommunication systems.