Abstract X‐ray detectors are used widely, but they suffer from short operational stability and insufficient absorptivity of X‐ray photons. Here, a strategy for room‐temperature, solution‐grown δ‐CsPbI 3 monocrystalline microwires exhibiting one of the highest linear X‐ray absorption coefficients among known semiconductors is reported. In a metal–semiconductor–metal architecture, δ‐CsPbI 3 demonstrates one of the lowest detectable X‐ray dose rates at 33.3 nGy air s −1 , enabled by its exceptionally low dark current density of 12 pA mm −2 . The detector remains stable following a continuous operation for a total accumulated dose of 1.44 × 10 6 Gy air , which is equivalent to 720 million chest X‐ray scans. The 1D crystalline nature of δ‐CsPbI 3 enables a high X‐ray imaging resolution of 12.4 line pairs per millimeter (lp mm −1 ).