Silver nanoparticles (AgNPs) synthesized with aqueous extracts of Pterocladiella capillacea presented antibacterial activity against Staphylococcus aureus. AgNP preparation was optimized, leading to elongated spheres with a mean diameter of 17.42 nm. The minimum inhibitory concentration (MIC) was 0.74 μg_Ag mL⁻¹, the lowest reported up to date for algae-stabilized AgNPs.