A rapid, sensitive and specific method for quantification of methoxsalen in human plasma using metaxalone as the internal standard is presented. An Applied Biosystems Sciex API 3200 triple quadrupole mass spectrometer in multiple reaction monitoring mode, using TurboIonSpray in the positive ion mode was used. Simple solid phase extraction was followed by C18 reverse phase chromatography and tandem mass spectrometry using metaxalone as the internal standard. The mean recovery for methoxsalen was 90.0% with a lower limit of quantification of 1.0 ng mL⁻¹. This validated assay method makes use of the sensitivity and selectivity of tandem mass spectrometry detection to allow for a more rapid and selective method for the determination of methoxsalen in human plasma.