The thermoelectric properties of a nonstoichiometric titanium oxide (TiO1.1) are investigated in terms of materials for high-temperature thermoelectric conversion. The electrical conductivity, σ, of TiO1.1 increases to 9000 S/m at 1073 K, is showing semiconducting behavior. The Seebeck coefficient, α, of TiO1.1 shows a general trend in which the value increases gradually from 0.4 mV/K at 573 K to 1.0 mV/K at 1223 K. As a consequence, the power factor, α2σ, reaches 8.6×10−3 W/(m·K2), the largest value of all reported oxide materials. The thermal conductivity, κ, of TiO1.1 increases with temperature, from 1.3 W/(m·K) at 573 K to 7.1 W/(m·K) at 1223 K. In spite of the considerably large values of κ, the figure of merit, Z = α2σ/κ, reaches 1.6×10−3 K−1 for TiO1.1 at 973 K. The extremely large power factor of TiO1.1 compared to other metal oxides can be attributed to the optimal carrier density. The dimensionless figure of merit, ZT, of 1.64 attained by TiO1.1 at 1073 K is the largest value of all reported other thermoelectric materials in this temperature region. And that TiO1.1 has ZT values of nearly unity or greater in the range of 773 to 1223 K, demonstrates the usefulness of the nonstoichiometric titanium oxides for high-temperature thermoelectric conversion.