High‐performance UV photodetectors call for sensitive and energy‐efficient signal detection in extreme environments. To satisfy the requirement of a UV detection without an external power consumption, self‐powered UV photodetectors must be realized by an optimal combination of heterostructure with maximum built‐in potential using novel wide‐bandgap materials. Here, self‐powered UV photodiodes are designed via the band engineering of a wide‐bandgap Sr(Sn,Ni)O3/BaSnO3 heterojunction for the first time. Based on the theoretical concept of acceptor doping by Ni substitution in SrSnO3, remarkably, this heterojunction with a conduction band offset of 0.94 eV shows strong nonlinear electrical characteristics with extremely low Idark (≈100 fA) owing to the spatial gradient of the potential barrier across the interfaces, outstanding photo‐to‐dark current ratio (>107 at 25 °C and > 104 at 300 °C), and high stability under various extreme conditions upon UV illumination even without external bias (V = 0 V). This study suggests a novel strategy that utilizes band engineering to maximize sensitivity and minimize energy consumption in harsh environments for UV imaging using the newly discovered wide‐bandgap semiconductors. High performance self‐powered np photodiodes have been demonstrated using perovskite stannate BaSnO3/Sr(Sn,Ni)O3 heterojunction. Outstanding photo‐to‐dark current ratio (> 107 at 25 °C) even at high temperature (> 104 at 300 °C) without an external power supply is achieved by exploiting excellent thermal/chemical stability of perovskite stannate and band engineering at the interface of np junction.