The paper presents a miniaturized diplexer based on a half mode substrate integrated waveguide (HMSIW) loaded with a novel metamaterial unit cell. The proposed diplexer is implemented by cascading two HMSIW bandpass filters loaded with modified circular complementary split-ring resonators (MC-CSRRs) operating at distinct frequencies. The working principle behind the operation of diplexer is the theory of evanescent mode propagation that aids in the miniaturization of the diplexer in addition to the 50 % size reduction contributed by the HMSIW. The performance of the diplexer is investigated by varying the substrate thickness and substrate material. To authenticate the proposed idea, the diplexer operating at 3.5/5.8 GHz is designed using the full-wave simulator, fabricated and experimentally measured. The proposed diplexer yields the advantages of low insertion loss, smaller footprint, reasonable isolation and individual control of operating frequencies. A second-order diplexer is designed and simulated that results in improved isolation and selectivity.