In the present study, red algae Portieria hornemannii was scrutinized for the productive synthesis of silver nanoparticles. These biosynthesized nanoparticles were characterized by visible color change and ultraviolet visible spectrophotometry that indicated the formation of nanoparticles at the absorbance of 418 nm. Fourier transforms infrared spectroscopy, X-ray Diffraction analysis was further carried out to study the functional groups and crystalline nature of the substance. Scanning electron microscopy and Transmission electron microscopy exposed the shape of the silver nanoparticles, which was found to be spherical. Energy dispersive X-ray spectroscopy confirmed the presence of the metal silver. Stability of nanoparticles was analyzed using Zeta potential. The synthesized nanoparticles were found to be active against the fish pathogens Vibrio harveyii, Vibrio parahaemolyticus, Vibrio alginolyticus and Vibrio anguillarum. The highest activity was found against the pathogens V. harveyii and V. parahaemolyticus. We have established an environmental friendly synthesis of Silver nanoparticles which can be used as an alternative to commercially available antibiotics in the treatment of fish diseases.