Toward conducting clinical pharmacokinetic studies of an antineoplastic agent, lenvatinib, we developed a liquid chromatography-tandem mass spectrometric assay for its quantitative analysis in human plasma. Analyte (lenvatinib) and internal standard (IS, propranolol) in the plasma were extracted by using acetonitrile and chromatographically separated by using a XTerra MS C18 column with 0.2 mL/min flow and mobile phase starting with 0.1% formic acid in water, followed by increasing percentage of acetonitrile. Detection was performed by using combined reversed-phase liquid chromatography-tandem mass spectrometry (LC/MS-MS) with positive ion electrospray ionization. MS-MS ion transitions used were 427.602>371.000 for lenvatinib and 260.064>116.005 for IS. This study was validated for accuracy, precision, linearity, range, selectivity, lower limit of quantification, recovery, and matrix effect according to the Guideline on Bioanalytical Method Validation in Pharmaceutical Development in Japan. Calibration curve was plotted by using lenvatinib concentrations ranging within 9.6-200 ng/mL, and correlation coefficients (r2) were in excess of 0.997. Intra- and interday accuracy ranged within 95.8-108.3% with mean recoveries of 66.8% for lenvatinib, and precision was <6.7% at all quality control concentration levels. Matrix effect analysis showed extraction efficiency of 15.7% for lenvatinib. Collectively, these findings demonstrate the feasibility of this method to evaluate kinetic disposition of lenvatinib.