The thermoelectric (TE) properties of SnO2 codoped with Sb and Zn were investigated. Polycrystalline ceramics of n-type SnO2-based semiconductors (Sn1−2xSbxZnxO2, x = 0, 0.01, 0.03, 0.05, 0.07) were prepared by reactive spark plasma synthesis followed by thermal treatment at a temperature of 1123 K. X-ray diffraction (XRD) analysis showed that most of the XRD peaks could be assigned to peaks of the pure SnO2 phase. The x = 0.05 and 0.07 samples showed the formation of a Zn2SnO4 secondary phase along with the main SnO2 phase. Microstructural analysis by scanning electron microscopy showed that the average grain sizes of the Sb and Zn codoped samples were larger than that of the undoped sample. The highest TE power factor in this study was 1.20 × 10−4 Wm−1K−2, which was comparable to the previously reported value for the sample synthesized by conventional normal pressure sintering at 1573 K. The synthesis method we describe here is very effective for the low-temperature synthesis of SnO2-based TE ceramics.