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In the past decades, toxic gas emissions have increased significantly owing to the rapid growth of industry and road transportation. Therefore, monitoring major pollutants, such as NO2, is crucial to protecting human health. The 2D materials that contain numerous adsorption sites and exhibit ultrahigh chemical reactivity can be used as sensor materials to detect these toxic gases. Herein, highly uniform, large‐area carbon‐incorporating hierarchical MoS2 nanobranches are synthesized by metal–organic chemical vapor deposition (MOCVD). An in situ carbon‐incorporation method that uses the carbon impurity present in the precursor as the seed during the MOCVD process is employed to form a hierarchical structure containing abundant adsorption sites. A gas sensor based on the resulting C‐MoS2 nanobranches contains many edge sites exhibits high adsorption energy, and consequently, has high NO2 gas sensitivity. Hence, this hierarchical C‐MoS2 gas sensor shows excellent sensing properties, exhibiting a device response of 1.67 at an extremely low NO2 concentration (≈5 ppb). The limit of detection of the gas sensor for NO2 is calculated to be low (≈1.58 ppt), further confirming its exceptional performance. Thus, the hierarchical C‐MoS2 nanobranches deposited herein provide novel insights regarding the properties of 2D materials and are highly suited for fabricating high‐performance NO2 sensors.