Euscaphic acid, a triterpene acid, exists ubiquitously in medicinal plants and demonstrates various pharmacological activities. This active compound is often used as a marker compound for quality control. Hitherto, the pharmacokinetic (PK) information is relatively scarce; therefore, it remains open to question whether the euscaphic acid reaches the target sites in the body at concentrations high enough for the claimed biological effects. A robust analytical method is prerequisite for obtaining enough PK information of euscaphic acid, which is useful for interpreting its pharmacological effects. In this study, we developed and validated a rapid liquid chromatographic tandem mass spectrometric (LC-MS/MS) method for the measurement of euscaphic acid in rat plasma. The rat plasma samples were precipitated with acetonitrile and the resulting supernatants were separated by a 4 min pulse gradient method on a Synergi Fusion-RP C₁₈ column (4 μm, 2.0 mm i.d. × 50 mm). Ursolic acid was used as an internal standard for quantification of euscaphic acid. Deprotonated euscaphic acid and its internal standard were generated in negative electrospray ionization (ESI) mode and their precursor-to-product ion pairs (m/z 487.4→469.3 and 455.5→455.4, respectively) were used for measurement. Notably, the commonly used concentration of formic acid (HCOOH; 1‰ and 5‰, v/v) in the mobile phase seriously suppressed the signal intensity, but this mobile phase additive at a much lower concentration level (0.1‰ and 0.2‰) could overcome the matrix effects and therefore increase the sensitivity of MS detection of euscaphic acid. The newly developed bioanalytical assay possessed favorable accuracy and precision with a lower limit of quantification of 2.0 ng mL⁻¹ and was successfully applied to PK studies in rats. The experimental strategies presented herein may be helpful for measurement of other triterpene acids in biological matrices.