The paper presents a simple one-step "green" approach to the synthesis of magnetite nanoparticles. The magnetite nanoparticles were synthesized using fruit extract obtained from grape peels and grape pulp. The formation of magnetite nanoparticles was confirmed by X-ray diffraction analysis (XRD), infrared spectroscopy (IR), Mössbauer spectroscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The crystallites sizes of magnetite nanoparticles are 7 and 14 nm for Fe3O4-peel and Fe3O4-pulp samples, respectively. Scanning electron microscopy images show that the samples consist of agglomerated nanoparticles. The synthesized magnetite nanoparticles showed good prospects for their use in magnetic hyperthermia. SAR values of 0.488 W/g and 1.330 W/g for Fe3O4-peel and Fe3O4-pulp samples, respectively. The synthesized magnetic nanoparticles show excellent colloidal stability in the aqueous solutions. The maximum hyperthermia temperatures are 42.28 °C and 42.48 °C for Fe3O4-peel and Fe3O4-pulp samples, respectively. Studies of the catalytic activity of magnetite were performed in decomposition of hydrogen peroxide in a batch mode. The degrees of H2O2 decomposition are 67.5% and 65.25% for the Fe3O4-peel and Fe3O4-pulp samples respectively. The high catalytic activity of the synthesized samples makes them promising candidates for the decomposition of hydrogen peroxide in wastewater disinfection.