Metal oxide nanoparticles (NPs) are receiving significant consideration for possible applications in optoelectronics, nano devices, nano sensors, information storage and nano electronics. The aim of the study to was investigate the preparation and characterization of CuONPs using extract of Dictyota dichotoma. Dictyota dichotoma extract was found suitable for the synthesis of copper oxide nanoparticles through green method. Spherical, polydispersity of CuONPs of particle sizes ranging from 12 to 52 nm are obtained. Changes of color occur due to surface plasmon resonance during the reaction with the ingredients present in the extract of Dictyota dichotoma resulting in the formation of CuO nanoparticles, which is confirmed by, FT-IR, UV–vis spectroscopy and SEM. FT-IR spectroscopic study confirmed that the phenol, alcohol, alkynes and aromatics features a strong binding ability with oxide, suggesting the formation of a layer covering oxide nanoparticles and acting as a capping agent to stop agglomeration and supply stability to the medium, the possible biomolecule like phenol, alcohol, alkynes and aromatics responsible for the bioreduction process.