Pará chestnut husk (Bertholletia excelsa) (PCH), an agro-industrial waste largely generated in Brazil, was employed as a low-cost and efficient biosorbent to remove the cationic dyes Crystal Violet (CV) and Methylene Blue (MB) from aqueous media. PCH presented an amorphous structure containing carboxylic acids, esters, ketones and aldehydes on the surface. Non-porous and irregular particles were also observed. For both dyes, the biosorption capacity was favored under acid conditions. Equilibrium was attained within 40 min at 25 °C with a PCH dosage of 0.5 g L−1. The biosorption kinetic curves were satisfactory explained by the pseudo-first-order model. The Freundlich model was best for representing the equilibrium curves. The maximum biosorption capacities were 83.6 and 83.8 mg g−1 for CV and MB, respectively. PCH was efficient for treating a simulated textile effluent containing several dyes and chemicals, achieving a color removal of 90%. In this way, PCH can be considered as an option for treating colored effluents containing textile dyes.