The proximate chemical composition of Sargassum horridum was determined monthly, including extract yields of alginate, crude fucoidan, and ethanolic extract. In addition, the biological activity (antioxidant and antibacterial) of the ethanolic extract and anticoagulant activity of the fucoidan were examined. Significant monthly variation was found in the chemical composition (moisture, protein, ash, crude lipids, crude fiber, carbohydrates, and gross energy), as well as in the alginate, fucoidan, and ethanolic extract yields. The maximum yield of crude fucoidan (13.5% dry weight) and alginate (21.4% dry weight) occurred in May. Crude fucoidan increased the clotting time at all concentrations tested during all months, based on activated partial thromboplastin and prothrombin time assays. The maximum yield of ethanolic extract occurred in April and August (6% dry weight). The ethanolic extract was evaluated for free radical scavenging activity using the 2,2-diphenyl-1-picrylhydrazyl method, with the highest reduction percentage in April (53%). Only the extracts from March, July and August showed slight antibacterial activity against Vibrio parahaemolyticus, V. alginolyticus, and Staphylococcus aureus. Sargassum horridum thus represents a potential source of carbohydrates, anticoagulant, antioxidants and antibacterial compounds. The best time of year for harvesting the alga is from May to July.