The secondary X-rays generated by the interaction of high-energy particles with scintillators can be converted into lower-energy excitons through thermalization, emitting light in the process. Capturing these secondary X-ray quanta efficiently is key to enhancing scintillation performance and boosting radiation detector sensitivity. Here we report a molecular design strategy using organic ligands to reclaim energy lost during the relaxation of secondary X-rays. This approach results in an enhancement in radioluminescence within lanthanide metal complexes by more than three orders of magnitude. By controlling the triplet energy of these ligands, we enable lanthanide centres to capture dark triplet excitons with near-unity extraction efficiency. These excitons arise from the absorption of secondary X-rays and transferred to the lanthanide centres through resonance energy transfer. This process delivers radioluminescence with orders of magnitude higher efficiency than existing organic or commercial inorganic scintillators. Tailoring metal centres and their coordination environments allows these organolanthanide scintillators to tune their spectra from ultraviolet to near-infrared, with lifetimes adjustable from tens of nanoseconds to hundreds of microseconds. These molecular scintillators enable high-resolution radiographic imaging and X-ray-mediated photodynamic therapy. Our findings not only unravel the link between scintillation performance and triplet exciton recycling but also lay the foundation for designing highly efficient organic scintillators that could revolutionize various fields.