A facile surfactant-assisted ion-exchange method has been developed to prepare rhombic dodecahedral Ag₃PO₄ sub-microcrystals which are ca. 250–600 nm in size through employing uniform Ag₂CO₃ sub-microrods which are ca. 250–450 nm in diameter and 1–2 μm in length as a precursor for the first time. A series of contrast experiments indicates that the dosages and types of surfactants and different anion sources have a great influence on the formation of the Ag₃PO₄ rhombic dodecahedra. Based on time-dependent experiments, a reasonable so-called precipitation–dissolution–recrystallization growth mechanism for the Ag₃PO₄ rhombic dodecahedra has been proposed. The as-prepared sample exhibits higher photocatalytic activity than both commercial TiO₂ (P25) and irregular Ag₃PO₄ particles toward degradation of rhodamine B (RhB) aqueous solution under visible light irradiation. The pseudo-first-order rate constant for RhB photodegradation by the rhombic dodecahedral Ag₃PO₄ (0.164 min⁻¹) is about 7 and 2.5 times that of P25 and irregular Ag₃PO₄, respectively.