Herein, a simple and green hydrothermal method is introduced for the synthesis of nitrogen-doped carbon dots (N-CDs) by using lentils as both a carbon and a nitrogen source. The interaction of as-prepared N-CDs with AgNPs was utilized for the colorimetric detection of thioridazine hydrochloride. The yellow color of AgNPs changed to pale orange in the presence of N-CDs as a result of their adsorption on the surface of AgNPs. Furthermore, the addition of thioridazine hydrochloride to N-CD/AgNP mixture led to a change of the solution color to red. This unique behavior was exploited to design a sensitive colorimetric method for the determination of thioridazine hydrochloride in the concentration range of 5.0–150 μg L⁻¹ with a detection limit of 3.0 μg L⁻¹. The established method was applied to the determination of this drug in human serum samples.