A hybrid diplexer combining planar transmission line(s) and a waveguide is disclosed. In one embodiment, a diplexer includes first, second, and third ports. The diplexer also includes a first signal path and a second signal path. The first signal path may be used to convey lower frequencies, and may be implemented using planar transmission lines. The second signal path may be used to convey higher frequencies, and may be implemented, at least in part, using a waveguide. The first signal path may be coupled between the first port and the second port, while the second signal path may be coupled between the first port and the third port. In one embodiment, the first signal path may implement a low-pass filter, while the second signal path may implement a high-pass filter.