A rectangular-to-circular mode combiner/divider is provided. In one aspect of the invention, a power combiner is provided that comprises a plurality of rectangular waveguide ports arranged in an integral arrangement. Each rectangular waveguide port is operative to operate in a rectangular mode. The power combiner is also includes a circular waveguide port operative to operate in a circular mode, and a transition body that couples the plurality of rectangular waveguide ports to the circular waveguide port. The transition body has an inner transition cavity and an outer body operative to convert radio frequency (RF) signals between the rectangular mode and the circular mode, and provide a combined output signal at the circular waveguide port from RF signals received at the plurality rectangular waveguide ports.