Brown seaweed (Sargassum polycytum) is a type of seaweed with the potential to serve as a source of nutrition and medicine. The composition of the pigments contained in it varies greatly. Seaweed samples were collected from the coast of Sumbawa, Indonesia. The seaweed was initially extracted by soaking it in a mixture of acetone:methanol (7 : 3, v/ v). To separate the pigment content in seaweed, after obtaining a dry extract, isolation and identification were performed using Thin Layer Chromatography (TLC) and High-Performance Liquid Chromatography (HPLC) methods. Isolation was performed through column chromatography followed by identification using UV-Vis Spectrophotometry. Antioxidant activity was carried out by DPPH method. The results of pigment identification using TLC showed the formation of 6 pigment spots, namely β-carotene, pheophytin, chlorophyll a, chlorophyll c, fucoxanthin and xanthophyll. Further identification was conducted using HPLC equipped with a photodiode array detector (PDA) which resulted in 5 peaks: Peak 1 corresponds to chlorophyll _c1, peak 2 to chlorophyll _c2, peak 3 to fucoxanthin, peak 4 to xanthophyll, and peak 5 to carotenoid. The isolation process with column chromatography produced 6 isolates: Isolate 1 is β-carotene, isolate 2 is pheophytin, isolate 3 is chlorophyll a, isolate 4 is chlorophyll c, isolate 5 is fucoxanthin, and isolate 6 is xanthophyll, as determined by spectrum pattern identification. The antioxidant activity of β-carotene IC₅₀= (476± 0.32) mg.L^-1, pheophytin IC₅₀= (720± 0. 32) mg.L^-1, chlorophyll a IC₅₀= (683± 0. 32) mg.L^-1, chlorophyll c IC⁵⁰= (582± 0. 32) mg.L^-1, fucoxanthin IC₅₀= (413± 0. 32) mg.L^-1, and xanthophyll IC₅₀= (521± 0. 32) mg.L^-1.