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Ga2O3, as an ultra-wide band gap material with physical properties such as large atomic number, low intrinsic carrier density, and high irradiation tolerance, has shown great potential in X-ray detection. This letter reports a self-powered X-ray detector based on vertical p-NiO/Ga2O3 heterojunction diodes (HJDs). Benefiting from the high quality and high built-in potential of the p-n heterojunction, the HJD detector exhibited a pronounced photovoltaic response to X-rays at 0 V with a sensitivity of 212 nC $\cdot $ Gy $^{-1}\cdot $ cm−2 and a fast response time of less than 0.02 s in transient X-ray response measurements, which was much faster than that of the reported Ga2O3 Schottky barrier diode (SBD) detector. The power-voltage (P-V) test indicated that the device conformed to typical photovoltaic characteristics with a maximum output power of 2.95 nW. Moreover, the HJD detector showed a good linear property to various X-ray dose rates from 0.0383 Gy $\cdot \text{s}^{-1}$ to 1.149 Gy $\cdot \text{s}^{-1}$ .