The spread of infectious diseases and the increase in the drug resistance among microbes has forced the researchers to synthesize biologically active nanoparticles. Improvement of the ecofriendly procedure for the synthesis of nanoparticles is growing day‐by‐day in the field of nanobiotechnology. In the present study, we use the extract of green alga Botryococcus braunii for the synthesis of copper and silver nanoparticles. The characterization of copper and silver nanoparticles was carried out by using UV‐Vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD), and scanning electron spectroscopy (SEM). FTIR measurements showed all functional groups having control over reduction and stabilization of the nanoparticles. The X‐ray diffraction pattern revealed that the particles were crystalline in nature with a face‐centred cubic (FCC) geometry. SEM micrographs have shown the morphology of biogenically synthesized metal nanoparticles. Furthermore, these biosynthesized nanoparticles were found to be highly toxic against two Gram‐negative bacterial strains Pseudomonas aeruginosa (MTCC 441) and Escherichia coli (MTCC 442), two Gram‐positive bacterial strains Klebsiella pneumoniae (MTCC 109) and Staphylococcus aureus (MTCC 96), and a fungal strain Fusarium oxysporum (MTCC 2087). The zone of inhibition was measured by the agar well plate method, and furthermore, minimum inhibitory concentration was determined by the broth dilution assay.