In this study, four Cannabis sativa-based materials—leaves (L), cleaned leaves (CL), seeds (S), and cleaned seeds (CS)—were evaluated for their capacity to adsorb methylene blue (MB) and other dyes from water. The physical and chemical properties of the adsorbents were characterized using a combination of Boehm titration, scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, and BET surface area and pore size distribution (PSD) analysis based on nitrogen adsorption–desorption isotherms. The removal of synthetic dyes, such as methylene blue, from aquatic environments is important due to their toxicity, persistence, and environmental impact. In this context, the use of plant-derived waste materials represents a sustainable and cost-effective approach to water treatment, contributing to the circular economy. The adsorption behavior of MB, used as a model cationic dye, was investigated through both kinetic and equilibrium studies. All tested materials exhibited very rapid adsorption kinetics, with equilibrium reached within 1 minute. Kinetic modeling revealed that the adsorption data for L and CL fitted well to the pseudo-first-order model, while the pseudo-third-order model best described S and CS. The adsorption isotherms were analyzed using multiple models, including Langmuir, Hill, and Jovanovic monolayer. The best fit was obtained with the Hill model for L, the Langmuir model for CL, and the Jovanovic monolayer model for both S and CS, indicating diverse adsorption mechanisms depending on the material. The maximum adsorption capacities (q_max) were 907.79, 892.67, 845.87, and 844.51 mg g⁻¹ for L, CL, S, and CS, respectively. Moreover, removal efficiency in real water samples ranged from 60% to 90%, underscoring the practical applicability of these biosorbents. The results suggest that Cannabis sativa residues could serve as efficient, fast-acting, and environmentally friendly materials for dye removal in wastewater treatment.