​This study explores the adsorption potential of hydrothermally treated waste derived algal biomass for removing methylene blue (MB) dye. Synthesizing a modified hydrochar through hydrothermal carbonization (150°C, 35 bar) followed by NaOH modification, we observed enhanced thermal stability and distinctive chemical changes. Optimal conditions were determined at pH 6 and 1 h contact time. Soluble salts with cations were identified as impacting adsorption efficiency, with increased interference for higher cation charges. Thermodynamic parameters (ΔG, ΔH, and ΔS) indicated a spontaneous and exothermic process, the calculated values (−5.417 to −6.907 kJ mol−1, −29.0 kJ mol−1, −73.8 J K−1 mol−1) aligned with this behavior. Adsorption isotherms favored the Freundlich model, revealing heterogeneous multilayer adsorption, with a maximum capacity of 97%. Kinetic studies supported the pseudo-first-order model. This detailed exploration provides insights into thermodynamics, kinetics, and the impact of adsorption parameters on MB removal, emphasizing the practicality of alkaline-modified hydrochar as an effective, sustainable adsorbent.​