BiI3 single crystals were grown by the physical vapor transport method. The repeated sublimation of the starting material reduced impurities in the BiI3 single crystal to sub-ppm levels. The detector was fabricated by depositing Au electrodes on both surfaces of the 100-μm-thick BiI3 single crystal platelet. The resistivity of the BiI3 single crystal was increased by post-annealing in an iodine atmosphere (ρ=1.6×1011Ωcm). Pulse height spectroscopy measurements showed clear peaks in the energy spectrum of alpha particles or gamma rays. It was estimated that the mobility-lifetime product was μeτe=3.4–8.5×10−6cm2/V and the electron–hole pair creation energy was 5.8eV. Our results show that BiI3 single crystals are promising candidates for detectors used in radiographic imaging or gamma ray spectroscopy.