Flower-like β-Bi2O3 microspheres with high specific surface area and excellent visible-light-driven photocatalytic activity (for degradation of Rhodamine B) were successfully synthesized via a facile hydrothermal process and subsequent calcination. By precisely adjusting the hydrothermal conditions, the composition and morphology of the microspherical precursors could be well controlled, so that upon further optimized calcination of the precursors, the selective formation of the monoclinic α-Bi2O3 and tetragonal β-Bi2O3 with three dimensional (3D) hierarchical architectures could be achieved. These tetragonal β-Bi2O3 microspheres with an average diameter of 3 μm were constructed by nanoflakes with an average thickness of 50 nm, which, as far as we know, is the first reported result on the 3D hierarchical architectures of tetragonal β-Bi2O3. Its flower-like microspherical architecture made the tetragonal β-Bi2O3 possess not only much improved specific surface area but also a narrower band gap, which significantly enhanced its visible-light-driven photocatalytic activity for the degradation of Rhodamine B (RhB). To further optimize the synthetic conditions and realize the controllable synthesis, the formation mechanism for the morphologies and polymorphs of the Bi2O3 microspheres was discussed in detail.