Constructing P–N heterojunctions is a well-known strategy to improve the gas-sensing performances of metal oxides. However, it remains a great challenge to design well-defined P–N heterojunctions, which hinders the achievement of high-performance P–N heterojunction gas sensors as well as the recognition of the P–N-heterojunction-enhanced gas-sensing mechanism. In this work, an α-MoO₃@NiO nanocomposite with well-defined core@shell P–N heterojunction nanobelts was prepared via a facile method. Benefitting from a well-defined P–N heterojunction construction, the α-MoO₃@NiO nanocomposite exhibited the highest response (R_g/R_a = 20.3) towards 100 ppm acetone, which was 17.2 times higher than that of pristine α-MoO₃ (R_a/R_g = 1.18) and 6.6 times higher than that of pristine NiO (R_g/R_a = 3.07). Gas-sensing measurement revealed that the response of n-type α-MoO₃ could be greatly improved by anchoring p-type NiO nanosheets. The main mechanism for the enhanced acetone sensing properties of the α-MoO₃@NiO nanocomposite was investigated. The well-defined α-MoO₃@NiO P–N nanocomposite offered abundant Mo–O–Ni bonds at the heterointerfaces, which could induce the large hole depletion regions in p-type NiO and hence benefit oxygen adsorption and gas-sensing reactions at the interfaces. This work not only demonstrates that the α-MoO₃@NiO nanocomposite is a promising material for fabricating acetone sensors but also provides useful guidance for designing well-defined core@shell P–N heterojunctions in gas-sensing applications.