Zero-dimensional (0D) halide perovskites have attracted extensive attention for their potential applications in solid-state lighting and X-ray detection due to their fascinating optoelectronic properties and convenient solution processability. Herein, we report the synthesis and photophysical properties of high-quality Sb³⁺-doped 0D Rb₂ScCl₅(H₂O) perovskite single crystals. The pristine crystals exhibit weak yellow self-trapped exciton (STE) emission peaking at 632 nm. The emission quantum yield can be dramatically enhanced from less than 1% to about 53% via Sb³⁺ doping. Spectroscopic characterizations indicate that the photoluminescence enhancement is a result of the efficient energy transfer from Sb³⁺ to the emissive STEs. Additionally, 0.2%Sb³⁺:Rb₂ScCl₅(H₂O) single crystals exhibit potential application in direct X-ray detection with a high sensitivity of 58.5 μC Gy^-1 cm^-2