In current trends of green synthesis of nanoparticles is rapidly shifting from plants based to marine algae as it is widely available as well as highly explore for pharmaceutical work. Presented work focuses on synthesising silver nanoparticles using marine algae Sargassum swartzii and its characterization through multiple authentic methods like UV-Vis spectrophotometer, X-Ray Diffraction, Fourier transform infrared spectroscopy, Scanning Electron Microscopy and Transmission Electron microscopes.The primary confirmation was done by the visible appraisal of the color difference from the light yellow-brown to dark brown colour. The UV-Visible absorbance spectra verified the formation of silver nanoparticles and spectra increased with incubation time. The Surface Plasmon Resonance (SPR) absorbance peak was observed at 439nm. SEM and TEM confirmed particles' surface morphology and size of AgNPs from 14 to 30nm. XRD approved particles' face-centric cubic and crystal structure and the size (15.33 nm) calculated through the Scherer equation. FTIR analysis reflected the various functional groups associated with the algal extracts, which help in the bindings of Ag molecules during AgNPs synthesis. The synthesized silver nanoparticles revealed significant antibacterial activity against Bacillus subtilis (27.17±0.73mm) and Staphylococcus aureus (23.53±0.29mm). The work reported that Sargassum swartzii widely available brown macroalgae, could be used as an alternative source for synthesis of AgNPs without destroying high plants and the produced AgNPs have efficient antibacterial activity against both gram positive and gram negative bacteria, which can be explore in curing several human diseases.