Van der Waals p–n junctions of 2D materials present great potential for electronic devices due to the fascinating properties at the junction interface. In this work, an efficient gas sensor based on planar 2D van der Waals junctions is reported by stacking n‐type and p‐type atomically thin MoS2 films, which are synthesized by chemical vapor deposition (CVD) and soft‐chemistry route, respectively. The electrical conductivity of the van der Waals p–n junctions is found to be strongly affected by the exposure to NO2 at room temperature (RT). The MoS2 p–n junction sensor exhibits an outstanding sensitivity and selectivity to NO2 at RT, which are unavailable in sensors based on individual n‐type or p‐type MoS2. The sensitivity of 20 ppm NO2 is improved by 60 times compared to a p‐type MoS2 sensor, and an extremely low limit of detection of 8 ppb is obtained under ultraviolet irradiation. Complete and very fast sensor recovery is achieved within 30 s. These results are superior to most of the previous reports related to NO2 detection. This work establishes an entirely new sensing platform and proves the feasibility of using such materials for the high‐performance detection of gaseous molecules at RT. An entirely new sensor platform is proposed based on an atomically thin van der Waals p–n junction of n‐type and p‐type MoS2. Compared to an n‐ or p‐type MoS2 sensor, the sensor based on the MoS2 p–n junction shows outstanding sensitivity, very fast recovery, and selectivity toward NO2 under UV irradiation at room temperature.