Thin films of a thiocyanate ion (SCN⁻)-doped organometal halide perovskite, CH₃NH₃PbI_3−x(SCN)_x, were used as a sensing material for developing high-performance gas sensors. The CH₃NH₃PbI_3−x(SCN)_x-based chemiresistor-type sensor can sensitively and selectively detect acetone and nitrogen dioxide (NO₂) at room temperature with high sensitivities of 5.6 × 10⁻³ and 5.3 × 10⁻¹ ppm⁻¹. The limits of detection for acetone and NO₂ were measured to be 20 ppm and 200 ppb. This sensor also exhibited excellent repeatability, and its environmental stability was greatly improved by doping the perovskite with SCN⁻ ions.