The waveguide multiplexer/demultiplexer includes a rectangular collecting waveguide provided with a short-circuit plane at one end thereof and elements for coupling the collecting waveguide to tunable waveguide filters arranged on opposing sidewalls, which are tunable to different frequency channels. The sidewalls are each provided with at least one pair of coupling openings. The two coupling openings of each pair are arranged side-by-side in a transverse direction relative to the waveguide longitudinal axis. Each coupling opening is spaced from the short-circuit plane at a distance corresponding to an odd-numbered multiple of a quarter mean wavelength for the frequency channel to be coupled via the respective coupling openings, and the two coupling openings of each pair are sufficiently spaced from the center of the sidewall transversely towards respective longitudinal edges of the sidewall so that one of the two waveguide filters for the two coupling openings can be coupled to the collecting waveguide via the coupling opening of the one waveguide filter when the two waveguide filters are tuned to different frequencies.