Seaweed extracts of Sargassum cinereum was used as a reducing agent in the eco-friendly extracellular synthesis of silver nanoparticles from an aqueous solution of silver nitrate (AgNO3 ). High conversion of silver ions to silver nanoparticles was achieved with a reaction temperature of 100 degrees and a seaweed extract concentration of 10% with a residential time of 3 h. Formation of silver nanoparticles was characterised by spectrophotometry and the scanning electron microscope. The average particles size was ranging from 45 to 76 nm. Antimicrobial activities indicate the minimum inhibitory concentration of biologically synthesised nanoparticles tested against the pathogen Staphylococcus aureus with 2.5 mewl (25 mewg/disc). High inhibitions over the growth of Enterobacter aerogenes, Salmonella typhi and Proteus vulgaris were witnessed against the concentrations of 100 mewg/disc. Promising potential and the future prospects of S. cinereum nanoparticles in pharmaceutical research are the highlights in this paper.