In this study, a simple and green synthesis of gold nanosheets was explained when red marine alga extract (Actinotrichia fragilis) collected from the Persian Gulf was used as both a reducing and shape controlling agent. Various nanosheet shapes such as a triangle, truncated triangle, hexagon and polygon were synthesized via this seaweed extract. The size, morphology and crystal nature of the as-prepared nanostructures were characterized by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and FT-IR spectroscopy. AFM analysis showed that the Au nanosheets have a thickness ranging from 10–15 nm. The as-prepared Au nanosheets were used as an electrocatalyst for the fabrication of an electrochemical sensor for oxidation of formic acid. The Au nanosheets modified carbon ionic liquid electrode (AuNS/CILE) was designed and its catalytic activity was investigated toward formic acid oxidation. The results imply that the AuNS/CILE shows excellent electrocatalytic activity and good stability for formic acid oxidation in a direct pathway without CO poisoning. These results, a direct pathway formic acid oxidation reaction without CO poisonous intermediates together with a simple and green route for the synthesis of Au nanosheets, show that the AuNS/CILE is one of the most promising systems for application in direct full cells.