The proposed study is the development of the RP-HPLC method for simultaneous quantification of tiagabine (TBN) and its related substance -A (TBN RS-A). The method was validated for its applicability both in bulk drug and tablet dosage form. For an isocratic elution, a mobile phase comprising of methanol: 0.1 mM acetate buffer mixture (65:45 v/v, pH 5.6) was used at 1 ml/min flow rate and ProntoSIL ODS C18 column (250mm × 4.5 mm; 5µm) column. At 240 nm as max, sharp peaks of TBN RS-A and TBN were observed at 3.7 and 5.2 min respectively. As per the ICH guidelines, the method was validated. Validation of the method was done as per the guidelines of ICH. Linear regression for the calibration curve was carried out for concentration ranges of 75-450 and 1-6 µg/mL respectively for TBN and TBN RS-A. The respective detection limits (LOQ and LOD) of the TBN and RS-A were found to be (2.014 and 0.032 µg/ml) and (6.103 and 0.098 µg/ml). The method was successfully extended for stability-indicating studies under different stress conditions.