Organic–inorganic metal halide perovskite (OIMHP) materials have been intensively investigated for optoelectronic applications, especially in solar cells. Here, we investigate OIMHP materials for gas sensor application to detect NO₂ gas, a harmful gas for humans and the environment. As a control, although the three-dimensional (3D) OIMHP NO₂ gas sensors exhibit good performance toward NO₂ gas when operating at room temperature, ambient stability remains the bottleneck for practical application. Using dimensional engineering of the OIMHP materials, not only the ambient stability but also the performance of the perovskite sensors has been greatly improved. The mixed-dimensional 2D/3D OIMHP gas sensors show an excellent sensitivity of 6.3 ± 0.83 times per ppm NO₂, good selectivity toward NO₂ gas, and quick response and recovery times of 5.7 s and 12.7 s, respectively. More importantly, the sensor performance can be stable under ambient conditions for almost two months, much longer than the control 3D-only device (13 days). The work paves the way for the development of perovskite based NO₂ gas sensors with superior stability and excellent performance at room temperature.