An ultrasensitive hydrogen sulfide gas sensor based on methylammonium lead perovskite is reported in this work. The pseudohalide molecular anion (SCN−) was introduced as a dopant in the perovskite-based active layer of the sensor. With the enlarged operational current provided by the vertical structure of the sensors, both the MAPbI3-x(SCN)x-based and the MAPbI3-based gas sensors exhibit excellent ppb-regime (∼200 ppb) hydrogen sulfide gas sensing capability. Particularly, with the SCN− doped, the MAPbI3-x(SCN)x-based gas sensor demonstrates an enhanced sensitivity and a much improved ambient air endurance by extending the device lifetime more than 5 days. Finally, we also conducted experiments to study the effects of relative humidity and light irradiation on sensing performance. Preliminary mechanisms were also proposed.