Air pollution is a global problem, which poses serious environmental concerns and health risks. In light of this, a key aspect of the challenge of managing air pollution is effective monitoring, which requires reliable high‐sensitivity sensors with strong selectivity and long‐term stability. Layered materials represent an emergent class of materials with extraordinary electronic properties and physicochemical properties, and, therefore, are a highly attractive prospect for this field. Here, such a sensor for NO₂ based on GaSe is presented. An ultrahigh sensitivity of 0.5 part per billion (p.p.b.) is achieved at room temperature, and the NO₂ selectivity ratios with respect to other likely interfering environmental gases are larger than 100. Moreover, no great degradation is observed after 10 days of air exposure. As a practical demonstration, the GaSe‐based sensors are also used to detect vehicle exhaust emission, and wearable …