Organic　scintillators　that　efficiently　generate　bright　triplet　excitons　are　of　critical　importance　for　high-performance　X-ray-excited　luminescence　in　radiation　detection.　However,　the　nature　of　triplet-singlet　spin-forbidden　transitions　in　these　materials　often　result　in　long-lived　phosphorescence,　which　is　undesirable　for　ultrafast　X-ray　detection　and　imaging.　Here　we　demonstrate　that　the　effect　of　hybridized　local　and　charge-transfer　(HLCT)　excited　states　enables　organic　scintillators　to　exhibit　highly　efficient　and　fast　radioluminescence　(RL)　in　response　to　X-ray　irradiation.　Our　experimental　and　theoretical　investigation　shows　that　the　oxidized　1,8-naphthalimide-phenothiazine　dyad　(OMNI-PTZ　2)　with　HLCT-excited　states　has　an　enhanced　overlap　integral　of　the　highest　occupied　molecular　orbital　(HOMO)　and　lowest　unoccupied　molecular　orbital　(LUMO)　on　MNI　π-orbitals,　and　moderate　donor-acceptor　electron　interactions.　As　a　result,　the　RL　of　these　crystals　exhibits　a　61-fold　increase　and　its　monoexponential　decay　lifetime　is　three　orders　of　magnitude　faster　compared　to　its　corresponding　thermally　activated　delayed　fluorescence　(TADF)　molecule　MNI-PTZ　1.　We　further　demonstrate　the　practical　utility　of　the　OMNI-PTZ　2　(G)　in　high-performance　X-ray　detection　and　imaging,　achieving　an　X-ray　dose　sensitivity　of　97　nGy　s−1　and　an　exceptional　spatial　resolution　of　20　lp/mm.　Our　study　provides　a　promising　molecular　design　principle　for　utilizing　triplet　excitons　to　develop　high-efficiency　and　fast　X-ray　scintillators　for　the　development　of　next-generation　flexible　and　stretchable　X-ray　imaging　detectors.　©　2024　Chinese　Chemical　Society.　All　rights　reserved.