A stability-indicating reverse-phase high-performance liquid chromatographic (RP-HPLC) method was developed and validated for the estimation of Metolazone in its bulk drug and pharmaceutical formulation. The method employed a Hypersil BDS C18 column (150 × 4.6 mm, 5 µm particle size) for chromatographic separation. The mobile phase consisted of acetonitrile and HPLC-grade water in a 50:50 v/v ratio, delivered at a flow rate of 0.7 ml/min, with detection at 236 nm. The method demonstrated linearity over a concentration range of 1–10 µg/ml, with a correlation coefficient (R²) of 0.9992. The limit of detection (LOD) and limit of quantitation (LOQ) were determined to be 0.1 µg/ml and 0.3 µg/ml, respectively. Statistical evaluation confirmed the method to be simple, precise, accurate, reliable, and reproducible, with %RSD for precision not exceeding 2.0%. The retention time for Metolazone was approximately 3.5 minutes. The percentage purity was found to be within acceptable limits. For stability studies, the drug was subjected to various stress conditions including acidic (0.1 M HCl), alkaline (0.1 M NaOH), oxidative (3% H₂O₂), and thermal (80°C) environments. Significant degradation was observed, particularly under acidic (89.6%) and oxidative (90.1%) conditions, followed by alkaline (91.9%) and thermal (94.5%) stress. The developed method proved to be suitable for the quantitative analysis of Metolazone in both bulk and dosage forms under various stability conditions.