In this study, a 3D thornball-like hierarchical β-In₂S₃, displaying extremely rapid photodegradation of cationic dyes, was synthesized by a facile method. The formation of a uniform thornball-like structure depended on the microwave reaction method and citric acid as the pH regulator. The size of In₂S₃ was easily adjusted by changing the microwave irradiation time from 5 min to 15 min. The morphology, structure, composition, energy level, charge separation, and surface properties of different-sized In₂S₃ were characterized. The results showed that In₂S₃ synthesized in 10 min (In₂S₃-10) displayed optimal interface property for the electron–hole separation, maximum hydrophilia with most surface negative charges for the surface adsorption, contributing to the complete photodegradation of rhodamine B (RhB) in just 25 minutes of visible light illumination. The photodegradation path of RhB was speculated with four possible paths, including the processes of de-ethylation, open-ring of xanthene, and rupture of carbon–carbon bonds up to the decomposition into small molecules. Finally, the reusability of In₂S₃-10 was tested, obtaining nearly 96% photodegradation efficiency after sequential 5 cycles.