The objective of this paper is to propose an adsorption isotherm model for methylene blue on composite activated carbons derived from cotton (Gossypium hirsutum) hulls and capsules. The adsorption mechanism involved was described using a three-parameter model that is intermediate between the Langmuir and Frundlich models and is characterized by its ease of use and physical interpretability. The experimental adsorption data were first adjusted using nine classical isotherm models, whose performance was evaluated using five statistical criteria: the adjusted coefficient of determination, the residual sum of squares, the Akaike information criterion (AIC), and the Bayesian information criterion (BIC). The models were best fitted in the following order: Sips˃ Toth˃ Redlich-Peterson˃ Khan˃Langmuir˃ Frundlich˃ Jovanovich˃ Dubinin-Radushkevich. The proposed model provides the best fit with the experimental data for methylene blue adsorption on activated carbons derived from cottonseed shells and cotton (Gossypium hirsutum) capsules, with a determination coefficient of 0.9910 and a theoretical maximum adsorption capacity qm of 820.86 mg.g-1, approximately double that predicted by classical models, illustrating an expected improvement in the predicted performance of the studied activated carbon.