In this paper, a novel three-port uniplanar microstrip line balun is proposed to transform balanced complex load termination to 50 Ω unbalanced port impedance. The even-odd mode analysis is carried out to derive the design theory, and a resistive structure is proposed to match and isolate the balanced ports terminated with complex impedance. To verify the design concept, a prototype working at 1 GHz is designed and tested, and the simulation and measurement results agree well with each other.