Nitrogen dioxide (NO2 )is a major air pollutant that causes degradation to both the environment and human health. Conventional nanostructured materials for NO2 sensing suffer from limitations such as high operating temperature, low selectivity, and an undesirable tendency to agglomerate during synthesis. In this work, a highly sensitive and selective NO2 sensor based on 3D molybdenum disulfide (MoS2)/reduced graphene oxide (rGO) composites is presented. The composites are prepared using a novel self-assembly/hydrothermal method, enabling a reduction of synthesis temperature. Gas sensing characterization confirms highly sensitive and selective detection of NO2. The fabricated sensor shows a response of 2483% toward 10 ppm of NO2 at 80 °C, which is a substantial improvement over recently reported MoS2/rGO prototypes. The proposed NO2 sensor achieves an ultra-low detection limit of 27.9 ppb. The unique sensing properties of the 3D networked MoS2/rGO composite form a basis for advanced NO2 sensing applications.