In the present study, sodium alginate, extracted from sargassum brown algae was used as a stabilizer and wrapping of copper (II) oxide nanoparticles resulting the formation of copper oxide-sodium alginate nanocomposite. For this purpose, sargassum brown algae was collected from the Persian Gulf off the coast of Bushehr and sodium alginate was extracted. CuSO4. 5H2O was used as a precursor of copper and H2O2 was used as a stimulus for the formation of copper oxide from copper sulfate. Confirmation of nanocomposite synthesis and study of their properties was performed by infrared spectroscopy (FTIR) analysis, X-ray diffraction (XRD), visible-ultraviolet (UV-VIS) absorption spectroscopy, scanning electron microscopy (SEM), scanning electron microscopy (SEM) TEM), and BET analysis. The TEM image of the nanocomposite showed that the thickness of the copper oxide nanoparticles was about 40 nm, which was well surrounded by a layer of sodium alginate with a thickness of 8 nm, which resulted in a very good dispersion of the particles. The X-ray diffraction analysis also confirmed the presence of copper (II) oxide nanoparticles. Antibacterial properties of the synthesized nanocomposites were studied by disk diffusion tests, minimum inhibitory concentration (MIC) and minimum lethal concentration (MBC) on Gram-positive bacteria Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pyogenes and Pseudomonas aeruginosa and Gram-negative Pseudomonas aeruginosa. Studies have shown that the highest antibacterial properties are related to Staphylococcus aureus with a diameter of 1.76. Also, the lowest MIC (2.08 µg/mL) was for Staphylococcus aureus and the highest MIC (16.66 µg/mL) was for Pseudomonas aeruginosa. In general, the results of the present study showed that the sodium alginate, isolated from the alga sargassum, can affect the size, morphology, reactivity and biomedical effects of the synthesized nanoparticles. Also, the catalytic activity of the synthesized nanoc