Timolol (TIM) is a potent β-adrenergic blocker, useful in treatment of ocular hypertension or open-angle glaucoma. Development and validation of stability indicating LCMS assay method for TIM was accomplished coherent with ICH guideline. Successful chromatographic separation of TIM with its four degradation products was attained by using gradient elution mode on reverse phase column using ammonium acetate buffer, pH 4.6 as mobile phase A and organic solvent as the mobile phase B. Chromatographic conditions were set such as 1.0 mL min−1 flow rate, 20 μL injection volume, 30 °C column temperature and 320 nm detection wavelength. Four major degradation products obtained from hydrolysis and photolysis, were identified and characterized with the combination of liquid chromatography–electrospray ionization mass spectrometry (LC–ESI/MS/MS) and accurate mass measurements. Degradation pathways were identified based on a comparison of the fragmentation pattern of the [M+H]+ ions of TIM and its degradation products. The method validation was performed as per ICH guideline Q2 (R1). © 2018 Elsevier B.V.