The economic exploitation of sisal (Agave sisalana) fibers leads to the generation of underutilized residue. Therefore, technologies must be developed to mitigate the environmental impacts caused by inadequate residue disposal and ensure more income from sisal exploration. The present study investigated the potential of agave residues in producing biochar with high adsorbent capacity via slow pyrolysis. Hybrid Itaporanga, Hybrid Bahia, and Mutant-1 hybrid agave varieties were precursors, and methylene blue (MB) dye was used as a solute. The agave residues presented similar contents of lignin (12.1–13.2%) and ash (10.3–13.9%), while the differences were recorded in the holocellulose values (31.4–42.4%) and extractives (23.2–33.4%). The increase in temperature favored an increase in the surface area and volume of micropores in the biochar; however, the adsorption capacity of MB dye was reduced. In isotherm experiments, the Freundlich model (R2 = 0.99–0.97) showed better fit than Langmuir (R2 = 0.96–0.85) and Temkin (R2 = 0.74–0.95) models. The pseudo-second-order models effectively adjusted the adsorption kinetics. The Hybrid Itaporanga biochar obtained at 400 °C outperformed the other biochars, offering static (80 mg g−1) and dynamic (180 mg g−1) adsorption values. Thus, thermochemical valorization of agave residue is attractive for obtaining a cheap and environmentally friendly adsorbent.Graphical abstract: