The nonlinear properties of electronic devices, such as diodes and transistors, can generate nonlinear responses such as harmonic and intermodulation for clutter rejection purposes. The naturally occurring clutter objects are generally linear, or they possess very small nonlinearity. They can be distinguished from man-made targets containing the above-mentioned nonlinear devices by exploiting nonlinear responses. In this paper, this nonlinear property was utilized for generating a 3 rd -order harmonic response for clutter rejection purposes. Existing nonlinear radars generally exploit 2 nd -order harmonic responses for clutter rejection purposes owing to their high power levels among the harmonic responses. However, due to their proximity to the fundamental tone, these radars require bulky and expensive filters and diplexers with steep roll-off for maintaining the linearity of the transmitter and receiver section of the radar. As the 3rd -order harmonic response and the fundamental tone are widely separated in the frequency spectrum compared to the 2nd order harmonic response, better isolation between the fundamental and harmonic response can be achieved. This results in relaxed requirements for filters and diplexers for these 3rd order based harmonic radars. Apart from that, the receiver and tag size would be reduced since size and frequency are inversely proportional. In this paper, the 3 rd -order harmonic radar and a passive tag were designed and operated in millimeterwave frequency bands, i.e., 24/72 GHz. Experimental validations were performed to prove their clutter rejection ability.