A sensitive, specific, selective and rapid LC-ESI–MS/MS method has been developed and validated for the quantification of defactinib in mice plasma using 13C3,15N-tofacitinib as an internal standard (I.S.). Sample preparation was accomplished through a liquid–liquid extraction process. Baseline chromatographic resolution of defactinib and the I.S. was achieved on an Atlantis dC18 column using an isocratic mobile phase comprising 0.2% formic acid in water and acetonitrile (25:75, v/v) delivered at a flow rate of 0.5 mL/min. Defactinib and the I.S. eluted at ˜1.59 and 0.99 min, respectively. The total chromatographic run time was 2.50 min. A linear response function was established in the concentration range of 0.13–106 ng/mL. Method validation was performed as per regulatory guidelines and the results met the acceptance criteria. The intra- and inter-day accuracy and precision were in the range of 5.57–13.3 and 8.63–12.1%, respectively. Defactinib was found to be stable under various stability conditions. This novel method has been applied to a pharmacokinetic study in mice.