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The development of high-quality GaN-based heterojunctions breaks through the limitation of lattice mismatch, which is of particular importance for promoting their optoelectronic applications. Herein, we report the incommensurate heteroepitaxial growth of single-crystal cesium lead iodine (CsPbI3) nanoplatelets on a c-GaN/sapphire substrate with a uniform rectangular shape and composition distribution via one-step chemical vapor deposition. The formed CsPbI3/GaN heterojunction presents a type-II band alignment with the valence and conduction band offsets of 2.12 ± 0.15 eV and 0.60 ± 0.15 eV, respectively. Compared with CsPbI3 grown on a mica substrate, the CsPbI3/GaN heterojunction exhibits remarkable photoluminescence quenching and a shorter exciton lifetime, which arises from the enhanced carrier separation and extraction efficiencies induced by the band-offset effect. Thanks to the complementarity between the band gaps of CsPbI3 and GaN, the optical absorption of the formed heterojunction is expanded from the visible light to ultraviolet. These findings suggest the large potential of the CsPbI3/GaN heterojunction for the development of high-performance integrated photovoltaic and optoelectronic devices. [ABSTRACT FROM AUTHOR]