This study aimed to investigate the kinetics, isotherms, and thermodynamics of biosorption of the cationic dye rhodamine B by a low-cost biosorbent prepared from Aspergillus oryzae cells. Culture medium composition (mineral salts, nitrogen source, and carbon source) influenced removal efficiency, and dye removal increased with increasing biosorbent concentrations until a plateau was reached at 10 g L −1 . Temperature and dye concentration were directly related to removal, and the highest removal efficiency was obtained at 40 °C and 200 mg L −1 of dye. The adsorption kinetics was best fitted to a pseudo-second-order model, and equilibrium data were well described by the Freundlich equation. Thermodynamic analysis indicated that the biosorption of rhodamine B by A. oryzae cells is physical in nature, spontaneous, and more favorable at higher temperatures and dye concentrations. Overall, the results suggest that inactivated A. oryzae biomass is a promising biosorbent for the removal of cationic dyes from wastewater.