Superparamagnetic iron oxide nanoparticles coated with natural polymers have found many applications in the field of biosensors as magnetic carriers due to the combination of two characteristics: the possibility of controlling the particles movement by applying external magnetic fields and attaching biomolecules to the particle through a chemical bond. The aim of this work was to prepare long-term stabilized particles with acid groups available to be used as magnetic carriers of biomolecules. In this study, we present the synthesis of maghemite nanoparticles coated with alginate, a natural polymer. Particles were characterized by electron microscopy. Magnetic properties were studied by vibrating sample magnetometry, which revealed the superparamagnetic behavior of maghemite nanoparticles. Horseradish peroxidase (HRP) was chemically bonded to the maghemite-alginate particles and the enzymatic activity of HRP was determined by a colorimetric technique. The maghemite-alginate particles were immobilized onto the surface of a gold electrode by means of a magnet and the HRP activity was followed electrochemically, showing that these particles can be successfully used in amperometric sensors. Furthermore, maghemite-alginate particles were also used for the diagnosis of the foot-and-mouth disease by means of an enzyme-linked immunoassay with electrochemical detection. 3ABC protein, a non-structural protein of the virus, was linked to the maghemite-alginate particles and used to selectively detect anti-3ABC antibodies in cattle sera.