Aim (For Research Use Only. Not for use in Diagnostic Procedures). In this work, we aimed to develop a method that utilizes a sensitive and fast hybrid triple quadrupole linear ion trap mass spectrometer in MS/MS/MS mode to determine THC-COOH in oral fluid with a LLOQ of 10 pg/mL. Methods A 750 μL volume of extraction buffer was added to the standards, and all samples, standards and controls were acidified prior to analysis. Solid-phase extraction was performed on all standards and controls prior to injection. The LC conditions included a Phenomenex Kinetex C18, 50 × 2.1 mm, 2.6 mm column with a methanol: water with 0.01% solvent combination for mobile phase at 0.5 mL/min flow rate and 40 μL injection volume. The mass spectrometer used was an AB SCIEX 6500 QTRAP LC/MS/MS system and was functioned in the MS/MS/MS mode. Results While THC-COOH can be analyzed with positive ionization at levels > 500 pg/mL, negative ionization was needed for high-sensitivity analysis. Excellent linearity down to 10 pg/mL was readily achieved in neat solvent, but initial studies showed poor linearity in the oral fluid/extraction buffer matrix. Multiple strategies were taken to resolve isobaric interferences from the analyte peaks including solid-phase extraction and the use of more selective MS3 fragment ions. In order to achieve low level detection the selectivity of the MS/MS/MS fragment ion was required to eliminate interferences and background. The method gives a LLOQ of 10 pg/mL for THC-COOH in oral fluid and is linear up to 1000 pg/mL and was tested successfully with donor oral fluid samples. It requires no derivatization and can be readily automated for high through-put analysis. Conclusion A high sensitive method was developed for the detection of THCA in oral fluid with a 10 pg/mL LOQ utilizing the selectivity of the MS3 experiment on a QTRAP system.