Development and validation of a challenging and multipurpose stability-indicating RP-UPLC assay and related substances method for the quantification of diclofenac sodium and its impurities in its injection, in the presence of its degradation products. Various stress conditions, e.g. acid, base, oxidation, thermal and UV radiation has been employed to assess the stability indicating nature of the method. The desired chromatographic separation was achieved on a reverse phase ACQUITY UPLC® BEH SHIELD RP18 50mm x 2.1mm, 1.7µm under isocratic condition with a short run time of 5 minutes. A mixture of Triethylamine buffer of pH 2.0 and Methanol was used as Mobile phase with a flow rate of 0.35mL/min. Quantitation was achieved by UV detection at 254 nm. The method was validated for Linearity, Accuracy, Precision, Specificity, LOQ and Robustness. A linear response (r2>0.99) was observed for Diclofenac and its impurity in the range of 0.2µg/mL – 230 µg/mL and 0.3µg/mL – 4.8µg/mL respectively. The Limit of Quantitation (LOQ) for diclofenac sodium and its impurity were 0.2μg/ml and 0.3 µg/ml respectively. The proposed method can be used in quality control for the determination of the diclofenac sodium and its impurity in its injection and for the stability studies as the method separates the drug from its related/degradation products and excipients.