On the basis of mode-matching techniques and numerical optimization, a novel X-band diplexer based on overmoded waveguides for high-power microwaves is presented in this paper. Two ridged overmoded circular waveguides are proposed as bandpass filters operating at TM01 mode and through mode-matching techniques, filters are designed with central frequency of 9.7 and 9.3 GHz and ±100-MHz bandwidth, respectively. The radial waveguide structure with six rectangular ports is used as diplexing manifold. Moreover, one radial waveguide structure placed between two filters efficiently extracts the microwave after filtering. The other radial waveguide structure is placed coaxially, which is connected with the first one through six rectangular waveguides and whereafter combines and outputs the microwave after extraction. For practical integration with two high power microwave (HPM) sources, a high efficiency 90 ° circular waveguide bend operating at TM01 mode is designed. Simulation results show that the designed diplexer can handle gigawatt-class HPM with transmission efficiency up to 97% for each channel in ±100 MHz.