This study focuses on the valorization of biomass waste from the fragrance industry in Provence, France. Adsorbents enriched with oxygen surface groups were produced from Lavandin (Lavandula genus) using the hydrothermal carbonization (HTC) process followed by further NaOH surface treatment. The original Lavandin hydrochar (LH) and NaOH surface treated Lavandin hydrochar (NaOH-LH) were systematically characterized by SEM, FTIR, elemental analysis, zeta potential, and Boehm titration. Specifically, the NaOH surface treatment scavenged the heterogeneous fragments adhering to the surface, increased the number of oxygen surface groups and optimized the ratio of these groups without damaging the hydrochar skeleton structure. Batch adsorption tests for the representative contaminant methylene blue (MB) showed that the adsorption process can be well described by the pseudo-second-order kinetic model and the Langmuir thermodynamic model. In neutral aqueous solution, the three tested adsorbents LH, 0.1%NaOH-LH, and 4%NaOH-LH had adsorption capacities of 52 mg·g−1, 137 mg·g−1, and 306 mg·g−1 for MB. Adsorption experiments allowed us to elucidate that the adsorption mechanism was dominated by electrostatic attractions, supplemented by hydrogen bonds, π-π interactions, and n-π interactions. At some conditions, these capacities overcame the surface density of available adsorption sites, which was attributed to the steric hindrance between stacked adsorbed MB molecules, as the main mechanism governing the maximal surface density of the adsorbed monolayer. This study proves that the combination of the HTC process with NaOH surface treatment method converts Lavandin biomass waste into an efficient adsorbent for MB, providing an economic and ecological alternative for its valorization.