A rapid, sensitive and specific HPLC–MS/MS method was developed and validated for the quantification of trabectedin in human plasma after using deuterated trabectedin as Internal Standard (IS). After the addition of ammonium sulphate, the analyte was extracted from human plasma with acidified methanol (0.1 M HCl). Chromatographic separation was done on an Accucore XL C18 column (4 μm; 50 mm × 2.1 mm) using a Mobile Phase (MP) consisting of CH3COONH4 10 mM, pH 6.8 (MP A) and CH3OH (MP B). The mass spectrometer worked with electrospray ionization in positive ion mode and Selected Reaction Monitoring (SRM), using target ions at [M−H2O+H]+m/z 744.4 for trabectedin and [M−H2O+H]+m/z 747.5 for the IS. The standard curve was linear (R2 ≥ 0.9955) over the concentration range 0.025–1.0 ng/ml and had good back-calculated accuracy and precision. The intra- and inter-day precision and accuracy determined on three quality control samples (0.04, 0.08 and 0.80 ng/ml) were <9.9% and between 98.3% and 105.3%, respectively. The extraction recovery was >81% and the lower limit of quantification 0.025 ng/ml. The method was successfully applied to study trabectedin pharmacokinetics in a patient with a liposarcoma who received 1.3 mg/m2 as a 24 h continuous i.v. infusion.