A fluorimetric/colorimetric mercury(II) sensor based on porphyrin-functionalized Fe₃O₄@SiO₂ core/shell magnetic microspheres has been developed and demonstrated by sol–gel grafting reaction. These multifunctional microspheres show excellent fluorescence sensitivity and selectivity towards Hg²⁺ over other metal ions (K⁺, Na⁺, Ba²⁺, Mn²⁺, Ca²⁺, Co²⁺, Cu²⁺, Ag⁺, Mn²⁺, Ni²⁺ and Pb²⁺). Upon addition of Hg²⁺, the color of porphyrin-functionalized Fe₃O₄@SiO₂ microspheres changes from red to green within 1 min and the fluorescence of microspheres becomes obviously weak. Conversely, no significant changes in fluorescence emission or color are observed in the parallel experiment with other metal ions. Regarding the reversibility of the microspheres, the color and fluorescence of the porphyrin-functionalized Fe₃O₄@SiO₂ microspheres in the presence of Hg²⁺ ion are found to be almost reversible when the microspheres are treated with EDTA solution. Furthermore, the used microspheres can efficiently remove Hg²⁺ ions in aqueous solution and easily separated from the mixture by adding an external magnetic field. Theses results suggest that functionalized Fe₃O₄@SiO₂ core/shell magnetic microspheres are potentially useful materials for simultaneously detecting and removing environmental pollutants.