This paper reports on the eco-friendly synthesis of gold nanoparticles (AuNPs) using the seaweed Sargassum polycystum C. Agardh extract. Biological synthesis for nanoparticle using plants is gaining considerable interest among researchers as an eco-friendly alternative to conventional physical and chemical methods, as this approach eliminates the use of toxic chemicals. Synthesized AuNPs was monitored by UV-Vis spectroscopy and was found to be complete within 30 min. Confirmation of elemental gold was carried out by elemental mapping using different physical techniques such as Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX), Scanning electron microscopy (SEM). The bio reduced AuNPs exhibited remarkably good anti-bacterial activity against pathogens specifically Pseudomonas aeruginosa (20 mm) which is more susceptible. The elaborate experimental evidences support that the seaweed Sargassum polycystum C. Agardh extract can provide an environmentally benign rapid route for synthesis of AuNPs that can be applied for various purposes.