Here stretchable, self‐healable, and transparent gas sensors based on salt‐infiltrated hydrogels for high‐performance NO2 sensing in both anaerobic environment and air at room temperature, are reported. The salt‐infiltrated hydrogel displays high sensitivity to NO2 (119.9%/ppm), short response and recovery time (29.8 and 41.0 s, respectively), good linearity, low theoretical limit of detection (LOD) of 86 ppt, high selectivity, stability, and conductivity. A new gas sensing mechanism based on redox reactions occurring at the electrode–hydrogel interface is proposed to understand the sensing behaviors. The gas sensing performance of hydrogel is greatly improved by incorporating calcium chloride (CaCl2) in the hydrogel via a facile salt‐infiltration strategy, leading to a higher sensitivity (2.32 times) and much lower LOD (0.06 times). Notably, both the gas sensing ability, conductivity, and mechanical deformability of hydrogels are readily self‐healable after cutting off and reconnection. Such large deformations as 100% strain do not deprive the gas sensing capability, but rather shorten the response and recovery time significantly. The CaCl2‐infiltrated hydrogel shows excellent selectivity of NO2, with good immunity to the interference gases. These results indicate that the salt‐infiltrated hydrogel has great potential for wearable electronics equipped with gas sensing capability in both anaerobic and aerobic environments. An intrinsically stretchable, self‐healing, transparent, and ion‐conductive hydrogel is utilized to fabricate NO2 gas sensors. The sensing mechanism reveals that the redox reaction of NO2 at the electrode–hydrogel interface induces current variation. The sensor exhibits high sensitivity (119.9%/ppm), ultralow limit of detection (86 ppt), good selectivity. Meanwhile, it can operate in both anaerobic and aerobic environments at room temperature.