In this study, a simple and rapid LC with tandem MS method was developed and validated for the simultaneous determination of kaempferol and quercetin in Thespesia populnea extract. The compounds were eluted using a Gemini C18 column (50 × 2.0 mm, 3 μm), with the mobile phase consisting of acetonitrile-0.3% formic acid in water at the flow rate of 0.400 mL/min. The assay exhibited a linear dynamic range of 25-2500 ng/mL for both kaempferol and quercetin. The values for intra- and interday precision and accuracy were well within the generally accepted criteria for analytical methods (<15%). Selectivity, linearity, LOD, LOQ, accuracy, and precision were evaluated for both analytes. The proposed method is accurate and sensitive and can be used for the routine quantification of kaempferol and quercetin in the herbal extract and in polyherbal formulations.