There is a budding need to develop a method for environmentally benign metal nanoparticle synthesis, that do not use toxic chemicals in the synthesis protocols to avoid adverse effects in medical applications. The present investigation dealt with the synthesis of copper oxide (CuO) nanoparticles from blue green alga, Spirulina platensis. The algal extract consisting of phytochemicals was used as the reducing agent and copper sulphate as the substrate. Synthesised nanoparticles were characterized by UV-Vis spectrophotometry, FT-IR spectroscopy, XRD and SEM. Antibacterial and anticancer activities were assessed for the CuO nanoparticles. The results indicated that the formed CuO nanoparticles were observed to be nanosheets. FT-IR spectral analysis elucidated the occurrence of biomolecules required for the reduction of copper oxide ions. The synthesized nanoparticles were found to be effective at the concentration of 1 mg/mL against Escherichia coli, Bacillus subtilis, Pseudomonas aeruginosa and Serratia marcescens. The cytotoxicity activity of CuO nanoparticle was evaluated by MTT Assay against colon cancer cell lines and confirmed that CuO nanoparticle at a concentration of 125 µg/mL had cytotoxic activity. In conclusion, the CuO nanoparticles were synthesized at a low energy supply, in an ecologically safe mode which could be utilized for pharmacological applications and various biotechnological studies.