A rapid and sensitive LC-MS/MS method for quantifying Armillarisin A in human plasma after a single oral dose (40 mg) has been developed and validated. Sample preparation used liquid-liquid extraction with a mixture of diethyl ether-dichloromethane (60:40, v/v) in an acidic environment. The retention times of Armillarisin A and the internal standard, probenecid, were 1.63 and 1.78 min, respectively. The calibration curve was linear over the range 0.15-50 ng/mL with a limit of quantitation of 0.15 ng/mL. The coefficient of variation as a measure of intra- and inter-day precision was <9.3% and the accuracy was in the range 92.5-108.0%. The Armillarisin A concentration-time profile in human plasma was determined after an oral dose of a 40 mg tablet.