Antifouling properties and stability in tissue fluids are crucial for the successful application of micro- and nanoparticles in biomedicine. In this study, we prepared monodisperse magnetic poly(glycidyl methacrylate) microspheres with amino groups (mgt.PGMA-NH2) by a multistep swelling polymerization of glycidyl methacrylate (GMA). This was followed by ammonolysis of oxirane groups and precipitation of iron oxides inside the particle pores to make the microspheres magnetic. To suppress nonspecific protein adsorption from biological media, the microspheres were covered by three generations of a compact amino acid dendritic network (Ser-Lys-Ser/Lys-Ser/Lys-Ser) using peptide chemistry. The resulting particles did not aggregate under physiological conditions and contained similar to 1 mmol of NH2/g that was available for further modifications. Alkyne groups accessible for click chemistry were introduced to the dendrimer-coated particles by a reaction with 4-pentynoic acid. The external particle surface and internal bulk were characterized by scanning (SEM) and transmission electron microscopy (TEM), atomic absorption (AAS), FTIR, X-ray photoelectron spectroscopy (XPS), and elemental analysis. Antifouling properties of the dendrimer and linear Ser-Ala-Ser/Ala-Ser/Ala-Ser peptide-modified mgt.PGMA-NH2 microspheres were challenged with solutions of proteins, such as bovine serum albumin (BSA), gamma-globulin (gamma-Gl), fibrinogen (Fg), and a mixture of them. Finally, a model azidealkyne cycloaddition reaction with I-125-radiolabeled azidopentanoyl-GGGRGDSGGGY(I-125)-NH2 (I-125-N-3-RGDS) peptide demonstrated that the dendrimer-modified particles are suitable for potential applications, including the separation of peptides and other biomolecules, diagnostics, mimetics, vaccine synthesis, etc.