The marine brown seaweed extract of Sargassum myriocystum was used for the synthesis of silver nanoparticles (AgNPs) which seems to be very simple and eco-friendly in nature. Synthesis of AgNPs was monitored by UV-visible spectrophotometer and showed a peak at 430 nm. The optimized nanoparticles were synthesized by changing of various parameters like pH, temperature and extract concentration of algae. The synthesized AgNPs using Sargassum myriocystum were characterized by X-ray diffraction assay (XRD), Fourier transforms infrared spectroscopy (FTIR) and scanning electron microscope (SEM) shows the crystalline nature, chemical groups responsible for nanoparticles synthesis and morphology respectively. The transmission electron microscope (TEM) images indicate that silver nanoparticles are more or less spherical and identical particle size with the average particle size is 25 to 65 nm. The antimicrobial activity of AgNP was analyzed against Bacillus subtilis, E. coli, Pseudomonas sp. and Klebsiella planticola. The antimicrobial activity was perceptibly observed from the zone of incubation. These results suggest that AgNP synthesized from Sargassum myriocystum can be used as constructive growth inhibitors and effective antibacterial material in assorted microorganisms which are useful in the varied medical device and antimicrobial control systems. Copyright © 2017 American Scientific Publishers All rights reserved.