Dammar-20S, 25R-epoxy-3β, 12β, 26-triol (26-OH-panaxadiol), the new sapogenin product of the oxidative cyclization of protopanaxadiol from ginseng, exhibits a significant pharmacological effect as an anti-tumor agent. A sensitive and rapid analytical method based on ultra-performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS) was initially developed for the detection of 26-OH-panaxadiol in rat plasma. Pre-treatment of the sample obtained from the plasma involved a single protein precipitation step using methanol. 26-OH-panaxadiol and an internal standard (IS), tanshinone II A, were separated on a Waters ACQUITY UPLC BEH C₁₈ analytical column (50 mm × 2.1 mm, 1.7 μm) using acetonitrile–0.1% formic acid in water (80 : 20, v/v) as the mobile phase, at a flow rate of 0.3 mL min⁻¹. Chromatography of the 26-OH-panaxadiol and IS was performed within 3 minutes. Detection was performed through positive ion electrospray ionization (ESI⁺) in multiple reaction monitoring (MRM) mode. The assay was linear over the concentration range of 1–1000 ng mL⁻¹ (r > 0.9960). The limit of detection (LOD) and the lower limit of quantification (LLOQ) were 0.3 ng mL⁻¹ and 1.0 ng mL⁻¹, respectively. The intra- and inter-day deviations (expressed as relative standard deviation, RSD) were ≤4.8% and ≤8.5%, respectively, and the accuracy (expressed as relative error, RE) was in the range of 4.5% to 5.3%. The recoveries of 26-OH-panaxadiol and IS were 91.2% and 98.7%, respectively, and the matrix effects were satisfactory in all of the biological matrices examined. This fully validated method was successfully applied to the pharmacokinetic study of rats after a single initial intragastric administration of 15 mg kg⁻¹ 26-OH-panaxadiol. The main pharmaco-kinetic parameters: T_max (the time to peak), C_max (the concentration to peak), and t_1/2 (the biological half life) were 3.08 ± 1.24 h, 2833.86 ± 1246.08 μg L⁻¹, 5.14 ± 1.01 h, respectively.