This paper presents the experimental results of the study of n-GaN/p-NiO heterojunction for application as a selective UV photodetector. Synthesis of n-type GaN layers was carried out by plasma-assisted molecular beam epitaxy (PA MBE) on GaN/c-Al2O3 template substrates. The p-type layers were formed by reactive DC magnetron sputtering of NiO films followed by annealing. Post-annealing in an oxygen atmosphere at a temperature of 550 °C was used to improve the crystallinity of the deposited NiO films. The optical and electrical characteristics of individual semiconductor layers and n-GaN/p-NiO diode structure were studied. Photoluminescence spectra of GaN layers showed the presence of a narrow peak near 3.43 eV. The optical band gap of the NiO layers, determined by the edge of optical absorption, was 3.35 eV. The study of the n-GaN/p-NiO heterojunction current-voltage characteristics under light and dark conditions showed the selective sensitivity of the diode structure to UV radiation. Furthermore, the manufactured structure demonstrated the behavior of a self-powered photodiode. At a wavelength of 365 nm, the detectivity of the photodiode was 6.8∙109 Jones and the photoresponsivity was 3.64 mA/W.