The present work is focused on the biosynthesis of the Silver (Ag) Nanoparticles using an aqueous extract of the green seaweed Halimeda gracilis. The visible colour change indicated the biosynthesis of Silver Nanoparticles and the specific peak produced within the UV-Vis spectrum confirmed the biosynthesis of Silver Nanoparticles. The possible functional groups were identified with Fourier Infrared Spectroscopy. The morphological characterization of biosynthesized Silver Nanoparticles was done by Scanning Electron Microscopy and Dynamic light scattering measurements and Zeta potential. The average size distribution of Ag-Nanoparticles were 295.9 (d.nm) and fairly stable with a zeta potential value of -28.6 mV. The size of biosynthesized Ag-Nanoparticles was also measured with X-ray diffraction assay. Due to agglomeration, the size difference of biosynthesized Ag-Nanoparticles in case of SEM and EDX occurred. The biosynthesized Ag-Nanoparticles were assayed for their antibacterial activity against some human pathogens and for their potential on seed germination of Abelmoschus esculentus and Raphanus sativus var. longipinnatus. The antibacterial activity of biosynthesized Ag Nanoparticles was the highest against Proteus mirabilis (2.33±0.2 cm), followed by Klebsiella pneumoniae, (1±0.0 cm). The effect of biosynthesized Ag-Nanoparticles on the seed germination of Raphanus sativus var. longipinnatus was excellent as the germination rate was 100 percent for Ag-Nanoparticles treated seeds, which was better than normal seaweed extract and seaweed liquid fertilizer treated seeds. The seed germination was also good for Abelmoschus esculentus with the treatment of seaweed mediated Ag-Nanoparticles as germination rate was 60 percent. This work proved that seaweed synthesized Ag-Nanoparticles are Phyto-friendly in nature and in future nano-bio fertilizer may be used as the growth promoter and eco-friendly Nano-bio-fertilizer.