Bi₇O₉I₃, a fresh member of the bismuth oxyhalide family, with hierarchical micro/nano-architecture is successfully synthesized by a one-step, template and surfactant-free solution method. The as-prepared product was characterized by various techniques. X-ray diffraction, X-ray photoelectron spectroscopy and thermogravimetric analysis confirm that the composition of the as-fabricated sample is Bi₇O₉I₃. Scanning and transmission electron microscopy observations reveal that the as-synthesized sample exhibits a microsized plate-like structure with dense nanosheets standing on their surfaces. The time-dependent morphology of the Bi₇O₉I₃ sample was investigated, and a possible formation mechanism of the hierarchical structure is proposed. More importantly, the Bi₇O₉I₃ exhibits an excellent photocatalytic activity in the degradation of phenol under visible light irradiation. The high catalytic performance of the Bi₇O₉I₃ hierarchical structure comes from its electronic band structure, high surface area and high surface-to-volume ratio. In addition, the Bi₇O₉I₃ hierarchical architecture is stable during the reaction and can be used repeatedly. The present work not only gives insight into understanding the hierarchical growth behaviour of complex bismuth oxide iodide structures in a solution-phase synthetic system, but also provides a new way to improve the photocatalytic performance by designing desirable structures and morphologies.