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Akaganeite, with the formula FeO(OH,Cl), has potential in the realms of environmental protection, energy conversion/storage, and biomedicine. However, its large-scale production is still challenging in terms of cost and environmental friendliness, which limits its commercial application. In this study, we report a one-step sonochemical fabrication method to address this challenge. Fabrication has been performed in aqueous FeCl3 solutions with the aid of acoustic cavitation by gas bubbles. The fabricated FeO(OH,Cl) nanoparticles (NPs) are porous and exhibit spindle shapes. With increasing sonication time, these NPs can be spontaneously assembled to form irregular micro-/nanostructures of varied sizes. When required, such assemblies could be suitably directed by adding appropriate templates, either natural or artificial. The template-directed assembly relies on the principle of minimum free energy and the resulting FeO(OH,Cl) NPs nucleating and growing along the added templates. This study demonstrates a sonochemical fabrication method amenable to large-scale production, that is inexpensive and environmentally friendly, providing monodisperse porous FeO(OH,Cl) NPs of spindle shape and their various assemblies for commercial purposes.