Promoting the oxygen reduction reaction (ORR) is critical for commercialization of intermediate-temperature solid oxide fuel cells (IT-SOFCs), where Sr₂Fe_1.5Mo_0.5O_6−δ (SFM) is a promising cathode by working as a mixed ionic and electronic conductor. In this work, doping of In³⁺ greatly increases the oxygen vacancy concentration and the content of adsorbed oxygen species in Sr₂Fe_1.5Mo_0.5−xIn_xO_6−δ (SFMIn_x), and thus effectively promotes the ORR performance. As a typical example, SFMIn_0.1 reduces the polarization resistance (R_p) from 0.089 to 0.046 Ω∙cm² at 800 °C, which is superior to those doped with other metal elements. In addition, SFMIn_0.1 increases the peak power density from 0.92 to 1.47 W∙cm⁻² at 800 °C with humidified H₂ as the fuel, indicating that In³⁺ doping at the Mo site can effectively improve the performance of SOFC cathode material.