In this paper, electromagnetic wave propagation and mode coupling in a chirowaveguide are treated using the coupled‐mode theory. A chirowaveguide, as defined in our previous work, is a conventional cylindrical waveguide filled with homogeneous chiral materials. A set of coupled linear differential equations is derived for various mode amplitudes in the waveguide. We then show that, in any single chirowaveguide, owing to the handed properties of chiral materials filling the waveguide, energy coupling occurs from one mode to the other. We also demonstrate that in a parallel‐plate chirowaveguide a TE mode can be completely converted into a TM mode and vice versa as they propagate in the guide. Thus a chirowaveguide can indeed be used as a mode converter. Selected results are compared with those reported in the literature. Applications of such mode coupling in the design of novel microwave, millimeter‐wave, and optical devices and components are mentioned.