In this paper, a solar-blind ultraviolet photodetector (PD) based on the graphene/vertical Ga₂O₃ nanowire array heterojunction was proposed and demonstrated. To the best of our knowledge, it is the first time that vertical Ga₂O₃ nanowire arrays have been realized. Ga₂O₃ nanowires were obtained by thermally oxidizing GaN nanowires grown by molecular beam epitaxy on n-doped Si substrate. Then, a monolayer graphene film was transferred to Ga₂O₃ nanowires to form the graphene/vertical Ga₂O₃ nanowire array heterojunction and transparent electrodes. The fabricated device exhibited a responsivity (R) of 0.185 A/W and rejection ratio (R258 nm/R365 nm) of 3×10⁴ at the bias of −5 V. Moreover, the fast response times of this PD were 9 and 8 ms for the rise and decay times under 254 nm illumination, respectively, which are attributed to the unique properties of nanowire arrays and the graphene/vertical Ga₂O₃ nanowire array heterojunction structure.