In this study, α-cellulose was extracted from lignocellulosic simpor leaf residue as a sustainable alternative to conventional cellulose sources. The extraction process involved the removal of hemicellulose, lignin, and other phytocompounds using alkali (NaOH) treatment and bleaching with hydrogen peroxide (H₂O₂). The nanocrystalline cellulose (NCC) was isolated from α-cellulose using sulfuric acid hydrolysis treatment followed by ultrasonication. The extracted α-cellulose and isolated NCC were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and dynamic light scattering (DLS). The obtained results confirmed that the extracted NCC exhibited characteristic cellulose functional groups and a crystallinity index of 64.7%, indicating the effective removal of amorphous regions through sulfuric acid hydrolysis. The thermal stability of the extracted cellulose increased to 332 °C due to the elimination of extractives. DLS analysis showed that the extracted NCC exhibited high colloidal stability in polar solvents, characterized by a zeta potential of −70.8 mV and an average particle size of 251.7 nm. This study highlights an environmentally friendly approach for converting low-value biomass waste into high-value cellulose materials with potential applications in sustainable packaging, biomedical applications and composite reinforcement.