Pentobarbital is a short-acting barbiturate with anticonvulsant and sedative-hypnotic properties. Pentobarbital may be used to induce sedation, control seizures, induce coma, and to manage patients with traumatic brain injury. In traumatic brain injury, use of pentobarbital is known to reduce both cerebral blood flow and oxygen consumption, which reduces intracranial pressure and cerebral ischemia. As there are currently no known commercially available methods for quantification, a quick and reliable liquid chromatography-tandem mass spectrometry (LC-MS/MS) method is described here for the quantification of pentobarbital. The analyte and internal standard, D5-pentobarbital, were extracted from human serum via solid phase extraction prior to LC-MS/MS analysis utilizing electrospray ionization (ESI) in negative ionization mode. Confirmation and quantitation of pentobarbital was performed using multiple reaction monitoring (MRM). The method was linear from 0.5 to 100 μg/mL with imprecision less than 5% coefficient of variability. Pentobarbital eluted at 2.61 min and the total run time was 7 min. The method only requires 100 μL of serum due to the high physiologic concentrations required for sedation/coma.