Heterojunction engineering plays an indispensable role in improving gas-sensing performance. However, rational heterojunction engineering to achieve room-temperature NO2 sensing with both high response and rapid recovery is still a challenge. Herein, a 2D/2D heterojunction of g-C3N4/SnS2 is designed to improve the sensing performance of SnS2 and used for ultrasensitive and rapid-recoverable NO2 detection at room temperature. The pristine SnS2 fails to work at room temperature because of its high resistivity and weak adsorption to NO2. After combination with g-C3N4 nanosheets, the g-C3N4/SnS2-based sensor exhibits an extremely high response (503%) and short recovery time (166 s) towards 1 ppm NO2 at room temperature. The improved sensing performance is primarily attributed to the increased adsorption sites and enhanced charge transfer induced by the 2D/2D heterojunctions with large interface contact area. This achievement of g-C3N4/SnS2 2D/2D heterostructures demonstrates a promising pathway for the design of sensitive gas-sensing material based on a 2D/2D heterojunction strategy.