A silicon-based photonic switch is proposed and simulated based on the multimode interference (MMI) principle and the free-carrier plasma dispersion effect in silicon-germanium. The proposed switch, designed for 1.55 μm window operation, is useful for DWDM optical networks. The switch consists of two input single-mode ridge waveguide ports, a MMI section, and three output single-mode ridge waveguide ports. In the MMI section, two index-modulation regions are placed to divert input optical signals from the two input ports to each of the three output ports. Switching characteristics are demonstrated theoretically by a beam propagation method for 1.55 μm operation. The simulated results show that the insertion loss of the switch is less than 1.43 dB, and the crosstalk is between -18 and -32.8 dB.