Lenalidomide, a potent immunomodulatory drug, is widely used in the treatment of myelodysplastic syndrome. To investigate its degradation behavior under stress and stability testing conditions, we developed and validated a novel stability-indicating ultra-performance liquid chromatography (UHPLC)-UV-MS tandem method with high specificity, precision, accuracy, and robustness. An Acquity UPLC Phenyl column (100 × 2.1 mm, 1.7 µm) was used for impurity profiling and quantification of lenalidomide at a detection wavelength of 254 nm, with an injection volume of 1.0 µL and controlled sample and column temperatures of 15 °C and 30 °C, respectively. The diluent consisted of 0.1 % formic acid and acetonitrile (90:10, v/v), while the mobile phases were 0.1 % formic acid (Mobile Phase A) and acetonitrile (Mobile Phase B). A 20-minute gradient elution at a flow rate of 0.2 mL/min was used for impurity analysis, whereas a 7-minute gradient at 0.3 mL/min was applied for assay determination. The method demonstrated good linearity for all analytes, ensuring reliable quantification. Stress and photostability studies revealed that lenalidomide was stable under high temperatures (105 °C for 10 days) and daylight/UV exposure but exhibited significant degradation under hydrolytic and oxidative conditions. Hydrolysis led to the formation of major degradation products A, B, and E, whereas oxidative stress conditions generated impurities C (-NH