Lead-free molecular one-dimensional (1D) perovskites are promising candidates for eco-friendly optoelectronic applications due to their strong quantum confinement and suppressed ion migration. Herein, we report the growth of centimeter-scale [(CH₃)₃NH]MnCl₃·2H₂O lead-free molecular 1D perovskite single crystals via a seed-induced crystallization method at room temperature. The crystal features edge-sharing octahedral chains forming isolated 1D core–shell quantum wires, and exhibits a direct bandgap of 4.99 eV, p-type semiconducting behavior, and warm red emission arising from self-trapped excitons. Planar X-ray detectors based on the single crystal demonstrate excellent performance, achieving a high sensitivity of 295μC Gy_air⁻¹cm⁻²at 50 V and an ultralow detection limit of 0.90 μGy_airs⁻¹, highlighting the potential of lead-free molecular 1D perovskites for efficient X-ray detection.