Next-generation wearable electronics requires mechanical robustness. In addition to the previously reported eco-friendliness, low cost, and light weight of molecular perovskites, flexibility is also a desired merit for their practical use. Here we design a flexible X-ray detector based on a novel molecular perovskite, DABCO-CsBr3 (DABCO = N-N′-diazabicyclo­2.2.2­octonium), which is the missing link between metal-free molecular perovskites A­(NH4)­X3 (A = divalent organic ammoniums) and conventional metal halide based ABX3 (B = divalent metal cations) perovskites. DABCO-CsBr3 inherits its band nature from A­(NH4)­X3, while it exhibits a stronger stopping power. DABCO-CsBr3 shows potential for high-performance ionizing radiation detectors due to low dark current, low ion migration, and an efficient mobility–lifetime (μτ) product. Finally, a molecular-perovskite-based flexible X-ray detector is demonstrated on the basis of the DABCO-CsBr3/poly­(vinylidene fluoride) composite, with a sensitivity of 106.7 μC Gyair –1 cm–2. This work enriches the molecular perovskite family and highlights the promise of molecular perovskites for the next-generation eco-friendly and wearable optoelectronic devices.