Pharmaceuticals are contaminants of emerging concern in aquatic environments, creating a growing demand for sustainable adsorbents derived from renewable resources. In this study, hydrochar adsorbents were prepared from fishery waste-derived chitin (CT) and chitosan (CS) by hydrothermal treatment at 150–250 °C to investigate the relationship between hydrochar properties and adsorption performance toward pharmaceuticals of environmental concern (Lin, X., et all. Bioresour. Technol. 2024, 414, 131583). CS hydrochars prepared at lower temperatures (150 and 200 °C) achieved hydrochar yields of approximately 90% and exhibited the highest adsorption capacities for carboxylate-containing pharmaceuticals owing to electrostatic interactions. Increasing the treatment temperature to 250 °C reduced the hydrochar yield by approximately 40%, decreased the density of available amino groups, and shifted the dominant adsorption mechanism from electrostatic interactions to hydrophobic and π–π interactions. In contrast, CT hydrochars consistently exhibited lower adsorption capacities than CS hydrochars because of less effective carbonization and a lower density of adsorption sites. The optimized CS hydrochar prepared at 200 °C for 4 h achieved equilibrium adsorption capacities of 54.1 mg g^-1 for ketoprofen and 48.4 mg g^-1 for clofibric acid. Furthermore, ultrasonic alkaline activation increased the adsorption capacities of the hydrochars by 5–40%. These results demonstrate that hydrothermal treatment conditions govern the structure–property relationships of marine biomass hydrochars, providing a practical strategy for converting fishery waste into efficient adsorbents for water purification.