In this study, Fe₃O₄@ZrO₂ magnetic core–shell nanoparticles (NPs) were synthesized and were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Fe₃O₄@ZrO₂ NPs were modified onto the surface of a magnetic glassy carbon electrode (MGCE) to prepare an electrochemical sensor (Fe₃O₄@ZrO₂/MGCE). The voltammetric behaviours of methyl parathion (MP) at the Fe₃O₄@ZrO₂/MGCE were studied by cyclic voltammetry. A sensitive and simple method for determination of MP was developed based on the strong affinity of ZrO₂ of the sensor to phosphoric moieties of organophosphate compounds (OPs). The electrochemical sensor was successfully applied to determine MP by square-wave voltammetry (SWV). Under optimal conditions, the cathodic peak current of MP showed a highly linear increase with MP concentrations over the range of 7.60 × 10⁻⁸ M to 9.12 × 10⁻⁵ M, with a detection limit of 1.52 × 10⁻⁸ M.