The graphitic carbon nitride (g-C3N4) nanosheets decorated three-dimensional hierarchical flower-like nickel oxide (NiO) composites (NiO/g-C3N4, Ni/CN) were synthesized via a facile hydrothermal method combined with a subsequent annealing process. The structure and morphology of the as-prepared Ni/CN composites were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and nitrogen absorption. The gas-sensing experiments reveal that the composites with 10 wt % two-dimensional g-C3N4 (Ni/CN-10) not only exhibits the highest response of 20.03 that is almost 3 times higher than pristine NiO to 500 ppm triethylamine (TEA) at the optimal operating temperature of 280 °C but also shows a good selectivity toward TEA. The gas-sensitivity promotion mechanism is attributed to the internal charge transfer within the p-n heterojunction. Furthermore, the high specific surface area of the Ni/CN composites promotes adequate contact and reaction between the composites and triethylamine molecules. Therefore, the Ni/CN sensor has a great potential application in detecting TEA.