Sinami (Oenocarpus mapora H. Karst), a native fruit found in the Amazon region of South America, has high nutritional value and is rich in lipids. However, the processing of sinami generates a large volume of agro-industrial waste, mainly composed of seeds. Our research comprises a proximal analysis of the sinami seed and its phenolic compounds and their antioxidant activity. The chemical analysis revealed high moisture but low protein, fiber, and lipid content. Furthermore, the extracts showed high in vitro antioxidant activity against 1,1-diphenyl-2-picrylhydrazyl radical IC50 (0.34 ± 0.001 mg/mL) and ABTS IC50 (0.10 ± 0.0002 mg/mL) free radicals. Based on this previous assessment, a gel exfoliant was developed. Since sinami seed powder is a novel ingredient, different formulations were evaluated to determine future incorporation into the cosmetic market. The best exfoliant gel prototype was studied under normal and stressed conditions (40 °C) for 3 months, maintaining a pH value of 5.25 and final viscosities of 700–800 mPa.s and 600–500 mPa.s under normal and stress conditions, respectively. Although unexplored, the sinami seed could be considered a raw material for the cosmetic industry.