AbstractN‐nitrosodiphenylamine (NDPhA) is soluble in water and highly toxic. Exposure to NDPhA even at low concentrations for a long period can cause several health effects. Hence the removal of NDPhA from aqueous samples using a widely abundant and economical adsorbent is highly challenging. In this study, a highly efficient, eco‐friendly, and low‐cost adsorbent was derived from the dead leaves of Halodule uninervis seagrass (SG) without any chemical modifications. The derived adsorbent was characterized and investigated for the removal of NDPhA from aqueous solutions. The effects of solution pH, contact time, adsorbent dosage, and adsorbate concentration were studied and optimized. The SG adsorbent showed a significant removal efficiency of 83% for NDPhA from wastewater solution using 1 g L−1 adsorbent. The maximum adsorption capacity for SG was found to be 64.84 mg g−1 at neutral pH and room temperature. The adsorption isotherm results fitted with the Langmuir model and followed pseudo‐second‐order kinetics. From the thermodynamic parameters, it has been found that the adsorption of NDPhA onto SG adsorbent is favorable, endothermic, and the chemical adsorption mechanism is predominant. This study indicated that SG adsorbent is reusable and can be easily regenerated and reused at least three times with a slight decrease in its removal efficiency.