Well-crystallized square-like bismuth oxychloride (BiOCl) nanoplates were successfully synthesized by a facile and environmentally friendly hydrothermal process in mannitol solution. The product was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), Raman spectroscopy, UV–vis diffuse reflection spectroscopy (DRS) and nitrogen adsorption. It was found that mannitol played a key role in the formation of square-like BiOCl nanoplates and the possible growth mechanism was also discussed. The photocatalytic activity of prepared BiOCl nanoplates was determined by the degradation of Rhodamine B (RhB) under visible light irradiation. The square-like BiOCl nanoplates exhibited excellent visible-light-driven photocatalytic efficiency, which was much higher than that of commercial BiOCl and TiO₂ (anatase). The remarkable visible-light photocatalytic activity was mainly attributed to the synergistic effect of the layered structure and the strong adsorption of RhB dye upon the BiOCl nanoplates, which might allow more efficient transport of the injected electrons. A possible dye-sensitized photocatalytic degradation process (photosensitization pathway) was proposed.