Zinc indium oxide films were deposited by the rf magnetron sputtering method using a (ZnO)3In2O3 target. The films were prepared at 573 K in various Ar/O2 sputtering gases (O2 content: 0%–25%). The effect of the oxygen gas content in the sputtering gas on the structural, optical, electrical, and thermoelectric properties of the films was investigated. The films had a c-axis oriented layer structure. The films deposited at 0%–3% oxygen gas contents exhibited a high electrical conductivity with a high carrier concentration, n≈1020 cm-3, while the conductivity of the films significantly decreased above the 3% oxygen gas content, having a carrier concentration below 1018 cm-3. From the optical transmission measurement, the band gap of the films was estimated to be 3.01 eV. The films deposited at 3%–8% oxygen gas contents showed a high Seebeck coefficient, -300 μV/K, while the maximum power factor, 4.78×10-5 W/m K2, was obtained at the 2% oxygen gas content. The Seebeck coefficient and the power factor were calculated on the basis of degenerate semiconductors. These results suggest that zinc indium oxide films have the possibility of being high performance thermoelectric materials.