Novel core-shell Fe3O4@poly(dopamine) magnetic nanoparticles were prepared through an in situ self-polymerization method. The hybrid nanomaterial showed an average core diameter of 11 ± 3 nm and a polymer thin film thickness of 1.8 ± 0.2 nm. The core-shell nanoparticles were employed as solid supports for the covalent immobilization of horseradish peroxidase (HRP), and the resulting biofunctionalized magnetic nanoparticles were employed to construct an amperometric biosensor for H2O2. The enzyme biosensor showed a high sensitivity of 442.14 mA M-1 cm-2, a low limit of detection of 182 nM, a wide linear range from 6.0x10-7 to 8.0x10-4 M and high stability for 1 month.