The cobalt ferrite (CoFe₂O₄) and silver-cobalt ferrite (Ag-CoFe₂O₄) nanoparticles were obtained through self-combustion and wet ferritization methods using aqueous extracts ofHibiscus rosa-sinensisflower and leaf. X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and magnetic measurements were used for the characterization of the obtained oxide powders. The antimicrobial activity of the cobalt ferrite and silver-cobalt ferrite nanoparticles against Gram-positive and Gram-negative bacteria, as well as fungal strains, was investigated by qualitative and quantitative assays. The most active proved to be the Ag-CoFe₂O₄nanoparticles, particularly those obtained through self-combustion using hibiscus leaf extract, which exhibited very low minimal inhibitory concentration values (0.031–0.062 mg/mL) against all tested microbial strains, suggesting their potential for the development of novel antimicrobial agents.