High-performance and energy-efficient ultraviolet photodetectors play a critical role in civilian and military fields. Here, patterned growth of highly crystalline straight Ga2O3 nanowires on p-type GaN epitaxial film wafer substrate is achieved by bottom-up technology using an economical chemical vapor deposition method. A self-powered ultraviolet photodetector based on GaN/Ga2O3 nanowires p-n junction for broadband UV detection is constructed by combining spin coating Poly(methyl methacrylate) as an insulating layer and magnetic sputtering an Au thin film as top transparent conductive electrode. This nanowires-film structure device exhibits excellent performance under zero bias voltage, including considerable responsivity of 960 mA W−1, high detectability of 3.82 × 1013 Jones, ultrahigh ultraviolet/visible inhibition ratio (R254 nm/R400 nm) of 7.92 × 103, fast rise/decay time of 4/12 ms and a high photo-to-dark current ratio greater than 104. The self-powered property under zero bias is attributed to the built-in electric field formed between the p-GaN film and the n-Ga2O3 nanowires, which creates a driving force for the rapid separation of photo-generated carriers. The high-performance mainly credits to the device's excellent UV light utilization efficiency and unique interfacial engineering of the p-n junction. This work provides a new avenue for the application of Ga2O3 nanowires in high-performance p-n junction self-powered photodetectors that can operate in harsh environments where it is inconvenient to transmit electric power.