A plasmonic frequency diplexer in a metal-insulator-metal waveguide at mid-infrared range is presented. The diplexer consists of two bandpass filters connected to a power splitter. A circuit-based model is used to successfully design the configuration. The effect of metal loss is carefully considered in our simulations. The performance of the designed diplexer is outstanding in terms of the frequency selectivity and loss. The proposed scheme is scalable in the infrared range and can also be used to realize frequency multiplexers.