An ecofriendly and efficient procedure for the synthesis of silver nanoparticles (AgNPs) was performed using aqueous extract of Enteromorpha intestinalis as reducing, capping, and stabilizing agents. AgNPs were characterized by UV spectroscopy, Fourier transform infrared (FTIR), high-resolution transmission electron microscope (HRTEM), and EDX. Moreover, the optimum conditions for the green synthesis were studied and the antimicrobial activity were estimated by agar well diffusion and broth dilution methods. UV vis spectroscopy confirmed the formation of AgNPs due to their surface plasmon resonance. The optimum conditions for the biosynthesis of AgNPs were using 2.5 g or 5 g/100 ml (w/v) of algal biomasses for extraction, and 1 mM of silver ions within 72 h. Transmission electron micrographs showed that AgNPs were spherical in shape and with a mean average size of 9.17 ± 3.2 nm. FTIR revealed the that protein and polysaccharides are responsible for AgNPs synthesis. Additionally, AgNPs exhibited antimicrobial activity against all tested microorganisms. The minimum Inhibitory concentration for all bacteria and Candida albicans ATCC26555 was 6.25 µg/ml except Klebsiella Pneumoniae ATCC70603 and Staphylococcus aureus ATCC4330 was 12.5 µg/ml. Further studies will be recommended to estimate the cytotoxicity of these AgNPs on the human cell line. The future use of algae as green nanofactory will be important in medical applications.