As being a good photoelectric and gas sensitive material, In₂O₃ is of great interest to the thermoelectric community due to its excellent thermoelectric properties at high temperature. In this study, the second phase InNbO₄ was successfully induced into the In₂O₃ matrix in situ by solid-state reaction combined with spark plasma sintering (SPS) to optimize the preparation process of bulk samples. It is found that introducing InNbO₄ distinctly affects the electrical transport properties of the In₂O₃/InNbO₄ composite samples, and its carrier concentration is dramatically increased. The highest electrical conductivity is 1548 S·cm^-1 at 1023 K, which is higher than those of most element-doped samples. The power factor of 0.67 mW·m^-1·K^-2 and the highest ZT value of 0.187 are achieved for the 0.998In₂O₃/0.002InNbO₄ sample at 1023 K. In conclusion, the electrical properties of In₂O₃ ceramics can be effectively improved by introducing in-situ InNbO₄ second phase, and thus the thermoelectric property at high temperatures is further regulated.