Synthesis of metallic nanoparticles using biological methods is more efficient and eco-friendly than chemical methods. In this study, we have investigated extracellular biosynthesis of silver nanoparticles using Sargassum anustifolium extract. The results recorded from UV–Vis spectrum, transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR) support the biosynthesis and characterization of silver nanoparticles using this seaweed. After adding silver nitrate to Sargassum anustifolium extract, the brownish yellow color in mixture turned in to dark brown color over time. The maximum peak absorption of synthesized silver nanoparticles was observed at 406 nm indicating the reduction of silver ions and formation of silver nanoparticles. The synthesized silver nanoparticles were predominately spherical shape and 32.54 nm size. According to the FT-IR analysis, the responsible compounds for reduction of silver ions into silver nanoparticles are phytochemical compounds of this seaweed.