Seven impurities in agomelatine drug substance were determined by a newly developed RP-HPLC method. Structures of potential impurities were confirmed by NMR and IR analysis. Efficient chromatographic separation was achieved on Hypersil BDS C18 column (250 mm × 4.6 mm, 5 μm) in gradient mode by using a binary mixture of potassium dihydrogen phosphate (15 mM, pH adjusted to 3.0) and acetonitrile at a flow rate of 1.0 ml/min. A photodiode array detector set at 230 nm was used for detection. Forced degradation studies showed that the proposed method was specific, and agomelatine was found to be susceptible to acidic and alkaline conditions. The method was validated according to ICH guidelines with respect to specificity, sensitivity, precision, linearity, accuracy, robustness and system suitability. Detection limit of impurities was in the range of 0.0008–0.0047%. Regression analysis showed correlation coefficient value greater than 0.999 for agomelatine and its seven impurities. Accuracy of the method was established based on the recovery obtained between 94.4% and 106.7% for all impurities. The validation results demonstrated that the developed method was suitable for the quantitative determination of potential impurities in agomelatine. A possible mechanism for the formation of impurities was proposed.