Brown algae (Phaeophyta) are multicellular eukaryotic organisms widely distributed in temperate and cold seas. To date, 1,836 species have been identified in approximately 285 genera. Most of them live in the marine environment where they play an important role, while less than 1% live in freshwater. Recent research has shown that brown algae are rich source of bioactive compounds with excellent nutritional value and are considered functional food with beneficial effect on human health. The aim of this thesis was to investigate the antioxidant potential of brown alga Halopteris scoparia. Algae samples were collected in the area of Čiovo island during the period of the alga growth season from May to September. Seaweed extracts were prepared using ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and a combination of both methods. The content of phenolic compounds in the extracts was determined using the spectrophotometric method by Folin-Ciocalteu. The antioxidant potential of H. scoparia algae was also investigated using the following methods: DPPH (2,2,- Diphenyl-picrylhydrazyl assay), FRAP (Ferric-reducing antioxidant power) and ORAC (Oxygen radical absorbing capacity). The results showed that the growing season influenced the antioxidant potential of this alga. Halopteris scoparia collected in May and extracted using the MAE method had the highest content of total phenolic compounds (631.67 ± 44.81 mg gallic acid equivalents/L) and the best antioxidant potential, with a result of 42.23 ± 0.78% inhibition measured by the DPPH method, 386.54 ± 37.17 μM trolox equivalents measured by the FRAP method and 96.19 ± 2.00 μM trolox equivalents measured by the ORAC method.