The castor bean plant (Ricinus comminis), is the only known source of the toxin ricin. Ricinine is an alkaloid present in the castor bean plant that can be used as a biomarker for ricin poisoning. There are only a few reports on the quantitative determination of ricinine in human’s biological samples including intoxication cases. In the present study, we developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method with simple protein precipitation for sample extraction. Using this method, we performed a quantitative determination of ricinine in postmortem blood of a woman who was dead in her house after taking several castor beans. The validation parameters including linearity, accuracy, precision, matrix effect, and recovery were satisfactory. The concentration of ricinine in the victim’s blood was 35.3 ng/mL. This method can be usefully used for the determination of ricin poisoning in forensic field and emergency medical treatment system.