Seaweeds are an enormous origin of biologically active components. Brown macroalgae are of tremendous interest at the moment because they contain a variety of bioactive substances that can be used for different therapeutic purposes. This study is focused on the one profusely available brown alga in the coastal area of Okha, Gujarat, Sargassum tenerrimum. Qualitative and quantitative phytochemical analyses were conducted for various bioactive compounds using different types of extracts. Further Fourier transform infrared spectroscopy (FTIR) and Reversed-phase high-performance liquid chromatography (RP-HPLC) analysis was conducted for confirmation of the functional group and the particular compound. An evaluation of qualitative phytochemical analysis revealed the existence of alkaloid, flavonoid, phenol, glycoside, tannin, terpenoid, steroid, coumarin and anthraquinones. Quantitative analysis of phytochemicals revealed that the S. tenerrimum. They contain a good amount of total phenolic content (68.13±2.55 mg GAE/gm), total flavonoid content (93.30±2.88 mg QE/gm) and total tannin content (93.13±2.8 mg TAE/gm) in the methanolic extract, which may have several antioxidant properties. The results based on FT-IR analysis revealed the presence of functional groups like alcohol, phenol, aliphatic, anhydrides, Aromatics, and Aliphatic amines. RP-HPLC analysis of methanolic extract confirmed the presence of gallic acid and quercetin. The presence of the phenolic compounds, flavonoids and several functional groups render good DPPH and HRSA activity, indicating this algal extract's medicinal properties. The conclusion put out is that Sargassum tenerrimum represents a vast source of biologically active substances with antioxidant potential that may be investigated for the pharmaceutical, cosmetic, and functional food sectors.