In this article, finite multilayer adsorption modeling was presented. The grand canonical formalism was used to establish a novel finite multilayer with multisite occupancy model. Expression for the physico-chemical parameters involved in the adsorption phenomena were derived based on statistical physics treatment. This model has been applied to one of the most challenging adsorption in liquid phase, i.e., Basic Bleu 41 dye adsorption onto raw and modified Posidonia biomass. The parameters involved in the analytical expression of the multilayer model such as the number of adsorbed molecules per site, the density of occupied receptor sites, and the number of adsorbed layers were determined by fitting the experimental adsorption isotherms at temperatures ranging from 303 to 353 K. Fitting results show that the dye molecules are multimolecular adsorbed onto Posidonia surface. Furthermore, the new approach leads us to quantify the mean number of adsorbed layers. The magnitudes of the calculated adsorption energy indicate that BB41 dye is physisorbed onto Posidonia adsorbent.