The authors have proposed and fabricated a vertical digital thermooptic waveguide switch with polymer. It consists of two vertically stacked waveguides, a vertically sloped waveguide to connect them, and a heating electrode. Its vertical switching operation is based upon the mode evolution effect in a vertical branch waveguide. The transfer characteristics of the device exhibit the digital response. The measured crosstalk is lower than - 13 dB and the power consumption in the heating electrode is about 800 mW. The switching time is about 7 Ins. This device will be useful to increase the integration density of planar lightwave circuits.