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Sisal fibers were employed as cost-effective adsorbent material to remove methylene blue (MB) and reactive black 5 (RB5) dyes from aqueous solutions. The fibers were characterized and the effects of adsorbent dosage and pH were investigated. Kinetic, equilibrium, and thermodynamics were analyzed. Desorption studies were performed and simulated textile effluents were treated with the fibers. The results revealed that the homogeneous surface diffusion model (HSDM) for cylindrical geometry was able to explain the adsorption kinetics, being the diffusion coefficients (DS) of 3.45 and 1.94 × 10−14 cm2 s−1 for MB and RB5, respectively. The BET model was adequate to represent the equilibrium data for MB and RB5. Adsorption capacities reached 553.4 and 310.2 mg g−1 for MB and RB5, respectively. Adsorption was spontaneous and favorable. Chemisorption was involved in the MB adsorption, while, physisorption was dominant for RB5 adsorption. Sisal fibers were efficient to decolorize a simulated effluent, reaching color removal percentage of 85%. It can be concluded that sisal fibers are a promising adsorbent to treat colored effluents, since has low cost, require little processing and possess high adsorption capacity for anionic and cationic dyes. [ABSTRACT FROM AUTHOR]