Nonstoichiometric orthorhombic titanium oxides (TiOx; x=1.86-1.94) prepared by reducing the anatase titanium oxide in H-2 at 950 degrees C are found to show large thermoelectric power. Their Seebeck coefficients are in the range -0.060 to -0.518 mV/K, and theoretical discussions are made based on the assumption that the electronic spin entropy dominating the entropy current is a main factor of the thermoelectric power. The figure of merit for TiO1.94 at 70 degrees C is estimated at 0.195. The nonstoichiometric orthorhombic titanium oxides, containing no hazardous elements, are promising materials for thermoelectric power generation.