Two-dimensional (2D) reduced graphene oxide–carbon dot (rGO–CD) hybrids were synthesized via a green one-pot method, in which the reduction of GO is accompanied by in situ generation of CDs with ultra-small sizes of 3–8 nm on its surface. The introduction of CDs enhanced the gas sensing properties by ∼3.3 times compared to bare rGO, and the rGO–CD hybrids could detect an extremely low NO₂ concentration (10 ppb) at room temperature. Upon exposure to 25 ppm NO₂ at room temperature, the as-prepared rGO–CDs show a sensitivity of ∼120% and exhibit excellent selectivity for NO₂. After six consecutive gas sensing tests or unencapsulated storage for 90 days, the rGO–CD hybrids still retained their sensing characteristics. The enhanced gas sensing properties of the rGO–CDs can be attributed to the increased hole density on the surfaces of rGO due to the introduction of CDs as well as the formation of all-carbon nanoscale heterojunctions with few residual N atoms, significantly promoting charge transfer. It is believed that this low-cost and environmental strategy provides a facile route to design and fabricate a high sensitivity, stability and repeatability NO₂ gas sensor based on 2D materials.