Sargassum sp. is a type of brown seaweed often found in tropical waters, but it has not been optimally used. The high cellulose content of Sargassum sp. can be used to produce cellulose nanocrystals (CNC). CNC can act as a bionanocomposite-reinforced nanomaterial. This study aimed to determine the most effective sulfuric acid concentration for extracting cellulose nanocrystals from Sargassum sp. CNC was extracted from Sargassum sp. using acid hydrolysis and sonication. The sulfuric acid concentration was varied to 30, 40, 50, and 60%. CNC was characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). FTIR analysis confirmed the presence of characteristic CNC functional group peaks, including C–O–C (~1160 cm-¹), C–O (~1050–1030 cm-¹), and β-(1→4)-glycosidic C–H (~897 cm-¹) as the CNC fingerprint. The FTIR findings indicated that the CNC extracted by sulfuric acid hydrolysis differed significantly from the raw Sargassum sp. material. Additionally, the XRD results showed that acid hydrolysis substantially affected the amorphous regions of cellulose. With 40% acid hydrolysis, the XRD analysis showed the highest CNC degree of 77.6%. Thermal analysis using TGA and DTG revealed that cellulose nanocrystals treated with 40% acid hydrolysis yielded CNC with enhanced thermal stability, exhibiting a maximum thermal decomposition temperature of 369.60°C. CNC isolated from Sargassum sp. cellulose has the potential to serve as a suitable source for manufacturing nanocomposites in various applications, such as pharmaceuticals, food packaging, and biomedical fields. © 2025, IPB University Department of Aquatic Product Technology. All rights reserved.