In the present investigation silver nanoparticles of spherical shape was synthesized biologically by rapid biological drip method using marine brown macro algae, Sargassum wightii (S. Wightii). Seaweed extract act as a reducing agent for silver nitrate. Silver nanoparticles have been characterized by UV-Visible spectrophotometer, Fourier- Transform Infrared spectroscopy (FTIR), X-ray diffractometer (XRD) and High Resolution Transmission Electron Microscope (HR-TEM). Antibacterial activity of these nanoparticles was carried out by antibiotic disc diffusion method against pathogenic microorganisms such as Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Klebsiella pneumoniae. Seaweed S. wightii mediated silver nanoparticles were found to be effective against these pathogens than silver nitrate.