The quest for highly sensitive and stable X-ray detectors has intensified, driven by diverse applications such as medical diagnostics and industrial inspection. Recent strides have been made in harnessing the potential of lead halide perovskites for radiation detection, thanks to their remarkable optoelectronic properties. However, the toxicity of Pb and intrinsic material instability restrict their practical applications as next-generation efficient detectors. Here, we have developed environment-friendly three-dimensional (3D) bismuth-chloride perovskites designed for highly sensitive and stable X-ray direct detection. By the introduction of monovalent Na⁺, one-dimensional (1D) Cs₃Bi₂Cl₉ can be converted into high-quality 3D double perovskite Cs₂NaBiCl₆ characterized by excellent photophysical properties. Our investigation, combining X-ray photoelectron spectroscopy and temperature-dependent photoluminescence, reveals a modulated electronic dimension and a mitigated electron–phonon coupling effect in Cs₂NaBiCl₆. As such, the 3D Cs₂NaBiCl₆-based direct detectors achieve a high sensitivity of 354.5 μC Gy⁻¹ cm⁻² and an ultralow detection limit of 59.4 nGy s⁻¹, with continuous on–off switching for 4500 s. Overall, our work provides a new direction for designing and developing lead-free perovskites to achieve highly sensitive X-ray detection with exceptional stability.