Bi₂WO₆:Er³⁺ with hierarchical flower-like microstructures were synthesized by a one-step hydrothermal method. The photocatalysts thus obtained were characterized in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS) as well as photoluminescence (PL) measurements. Results obtained by XPS confirmed the presence of Er³⁺ dopants in Bi₂WO₆. Moreover, the results obtained from SEM have shown that doping with Er ions results in hierarchical flower-like microstructures of Bi₂WO₆. Under visible light irradiation, with an appropriate doping content, the photocatalysts exhibit significant improvement in the photocatalytic activity as the separation efficiency of the photogenerated electron–hole pairs is enhanced. Notably, the photocatalytic activity of 1.5% Bi₂WO₆:Er³⁺ is 3.69 times that of undoped Bi₂WO₆. The results indicate that an appropriate doping content can improve photocatalytic activity, caused by wider band gap, hierarchical flower-like microstructures, and defects generated by doping with Er³⁺, which in turn increase the separation efficiency of the photogenerated electron–hole pairs.