Three taxonomically different seaweeds, Caulerpa prolifera, Sarconema filiforme and Padina tetrastromatica, were tested for silver nanoparticles (Ag NPs) fabrication. Different extraction methods were applied in addition to different concentrations of silver nitrate were added to the algal extracts. Ag NPs synthesis was monitored visually and by UV-Vis Scan and the chemical composition of the efficient extracts was considered. Antibacterial activities of the synthesized Ag NPs were tested against Escherichia coli (NCMB 11943) and Bacillus subtilis (ATCC6633). The results revealed the effect of both the extraction method and the precursor concentration on the synthesized Ag NPs. The studied extracts contained different biochemical compositions (quantitatively and qualitatively) which affect the synthesis process. Ag NPs synthesized using Padina showed high efficiency against the tested bacterial strains while Ag NPs synthesized using Sarconema were weak. Soaking extract of Padina synthesized dense Ag NPs with a mostly spherical shape, size ranging from 5 to 25 nm and Zeta-potential insinuated stability while Ag NPs synthesized using Sarconema had low counts. This study suggests that Padina tetrastromatica is a low-cost, green, efficient factory for Ag NPs synthesis compared to other taxonomically different seaweeds.