Dengue Fever is an infectious disease caused by dengue virus infection and it poses a significant threat in tropical countries, such as Indonesia. Several studies have shown that the existing treatment options for this condition were ineffective, including the Dengvaxia vaccine released in 2015. This has led to the exploration of bioactive substances with the potential to be developed as antiviral medicines. One of the promising candidates is brown seaweed (Sargassum plagiophyllum), which has been reported to have anti-dengue and cytotoxic effects. Therefore, this study aimed to assess the anti-dengue activity of fucoidan from S. plagiophyllum and analyze its monosaccharides composition. Fucoidan extract was purified with ion exchange chromatography and characterized with H-NMR and HPLC. According to HPLC analysis, the extract consisted of various sugars, including fucose (5980 g/g), galactose, xylose, mannose (1740 g/g), glucose (9.34 g/g), glucuronic, and galacturonic acid (10,06%). Furthermore, the anti-dengue activity was assessed using the viral ToxGlo test and the MTT assay. The results showed that fucoidan had an effective concentration (EC50) of 76.49 g/mL and a selectivity index (SI) of 3.02, indicating the presence of anti-dengue properties.