A series of porous BiOI nanoparticles were successfully synthesized via a hydrothermal process. The BiOI nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-vis absorption/reflection spectroscopy (UV-vis). The results showed that the as-prepared BiOI sonocatalyst generally had flake-like-shaped, porous particles about 100 nm in diameter. The sonocatalytic activity of BiOI was evaluated by the degradation of methylene blue (MB) in aqueous solution under ultrasonic irradiation. The effects of the catalyst dosage and initial MB concentration on the sonocatalytic degradation efficiency of MB were studied. The sonocatalytic degradation kinetics of MB was also investigated. The results showed that the degradation kinetics of MB fitted pseudofirst-order kinetics and the Langmuir-Hinshelwood model.