Pathogenic bacterial contaminations in water cause serious or even lethal threats. Strategies that effectively kill bacteria without causing environmental contamination are urgently needed in a wide range of applications. We prepared recyclable antimicrobial magnetic nanoparticles, Fe3O4@P(St-co-AcQAC), through surfactant-free seeded emulsion polymerization involving a polymerizable, hydropho-bic quatary ammonium compound (QAC). Fe3O4 particles were first synthesized by a solvothermal reaction, followed by functionalization with a methacrylic silane (MPS), and then copolymerized with a QAC-containing acrylic monomer (AcQAC), leading to Fe3O4 @P(St-co-AcQAC) nanoparticles. As confirmed by antibacterial assays, these Fe3O4@P(St-co-AcQAC) nanoparticles exhibited strong antimicrobial action against both Gram-positive Staphylococcus epidermidis and Gram-negative Escherichia coli, without leaching out any bactericidal agent. An additional benefit of antimicrobial magnetic particles is that they can be easily recycled while maintaining excellent antimicrobial efficacy.