A compact reconfigurable circuit performing as either Single Pole Double Throw switch or power divider is proposed and demonstrated, based on the concept of modes match and impedance match between substrate integrated waveguide (SIW) and half-mode SIW (HMSIW). A switchable HMSIW unit is first proposed and then followed by the design of the reconfigurable circuit. For demonstration, the proposed circuit is designed and fabricated on the substrate of Rogers 5880LZ with thickness 1.026 mm. When the circuit works as a switch and Port 2 is at ON-state, the measured S-11 < -15 dB, S-22 < -13 dB, S-21 > -1.5 dB, and S-23 and S-13 < -27 dB; meanwhile, when the circuit performs as a power divider, the measured S11 < -14.5 dB, and S-21 and S-31 > -4.35 dB, over the frequency band from 4.6 to 5.3 GHz. The good performances show the advantages of proposed circuit topology, which can be an excellent candidate for multifunctional microwave or millimeter-wave circuits.