Abstract Due to the limits of the physical properties of conventional semiconductors against harsh environments, seeking a suitable material for next‐generation photoconversion devices with high‐temperature stability and strong radiation hardness has become a hot issue. Here, visible‐light photodetectors are fabricated on an N‐doped diamond. Their visible‐light detection via charge‐neutralized impurity levels including multi‐complex mid‐gap states induced crystal defects shows photosensitivity of at least four orders (10 4 ), the visible responsivity of 0.08 A W −1 , and the detectivity of 3.9 × 10 12 Jones. No significant deterioration of such figures of merit of photodetectors is observed even at an environmental temperature of 250 °C and after absorption to 10 MGy in the dose of white X‐ray. N‐doped diamond shows excellent potential to be applicable to visible‐light photodetectors for harsh‐environmental implementations, which conventional visible‐light photodetectors are not capable of so far.