An analytical method was established for identification of degradation products of letermovir using UPLC-MS/MS. Letermovir was subjected to hydrolysis, oxidation, photolysis, and thermal stress according to ICH conditions. The analyte showed degradation under oxidative and photolysis conditions. Separation was done on an Intersil ODS column (150 × 4.6 mm; 5 µm) with water-methanol (20:80%, v/v) as mobile phase at 0.6 mL/min. Linearity was 50-500 ng/mL, with inter- and intra-day precision <15% and accuracy 97.07–101.98%. Degradation products were characterized using collision-induced dissociation mass spectral data. © 2020 Elsevier B.V., All rights reserved.