Objectives: The aim of this study was to establish an ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method for the detection of osimertinib in rat plasma, lung and brain tissues. Methods: Forty-eight rats were randomly divided into an experimental group (receiving osimertinib at doses of 5, 8, and 10 mg/kg) and a control group. After continuous intragastric administration for 15 days, samples of blood, lung, and brain tissue were collected. Chromatographic separation was achieved using a BEH C18 column with gradient elution, employinga mobile phase of water (containing 0.1% (v/v) formic acid) and acetonitrile. The concentration of osimertinib in the samples was determined using an AB SCIEX 5500 triple quadrupole mass spectrometer operated in positive electrospray ionization (ESI+) and multiple reaction monitoring (MRM) mode. Results: A UPLC-MS/MS analytical method for determining osimertinib concentrations was successfully established and validated. A linear relationship was observed for osimertinib concentrations in plasma within the range of 1-300 ng/mL, and in lung and brain tissues within the range of 0.5-50 ng/mL. The selectivity, accuracy, precision, matrix effect, extraction recovery, and stability all meetthe requirements of methodological validation criteria. Conclusions: A rapid and sensitive UPLC-MS/MS method was developed and validated for quantifying osimertinib concentrations in rat plasma, lung, and brain tissues, providing a valuable tool for pharmacokinetic and tissue distribution studies.