Er3+-doped BiOI porous microspheres were synthesized by an ethylene glycol-assisted solvothermal method. The TEM and HRTEM results revealed that all the synthesized microspheres were composed of ultrathin nanosheets with a thickness of ca. 4 nm and exposed negative {001} facets. These specific surface structures are not only beneficial for the adsorption of dye molecules but can also provide numerous active sites for photocatalytic reaction. In addition, the doping energy level of Er3+ ions can promote the separation of photogenerated electron-hole pairs. Thus, the as-synthesized 2 % Er3+-doped BiOI porous microspheres exhibited excellent performance for the removal of single as well as mixed dye molecules (anionic methyl orange and cationic Rhodamine B and methylene blue) under visible-light irradiation. The enhancement in the removal of dyes with different molecular structures by Er3+-doped BiOI ultrathin nanosheets can be attributed to the synergetic effects of surface adsorption and photodegradation. Possible photodegradation mechanisms of those dye molecules over Er3+-doped BiOI ultrathin nanosheets were discussed on the basis of the obtained results.