Edoxaban is mainly enzymatically converted to a 4-carboxylic acid form (4CA-EDX) and an N-desmethyl form (ND-EDX) in humans. This study aimed to establish a simple liquid chromatography-tandem mass spectrometry method using core-shell octadecyl silica (ODS) microparticles for the simultaneous quantitation of edoxaban and its two major metabolites in human plasma. Analytes extracted from plasma specimens by a one-step deproteinization were separated using a 2.6-µm core-shell ODS microparticulate column and linear acetonitrile-ammonium acetate gradient elution at a flow rate of 0.25 mL/min with a run time of 7 min. The mass spectrometer was operated in the positive ion multiple reaction monitoring mode. Plasma samples collected from 20 patients with atrial fibrillation were analyzed by the present method. The chromatograms of drug-free human plasma had no interfering peaks. The calibration curves of edoxaban, 4CA-EDX, and ND-EDX were linear over the concentration ranges of 1.25-160, 0.47-60, and 0.12-15 ng/mL, respectively. Their pretreatment recoveries and matrix factors were 88.7-109.0% and 87.0-101.6%, respectively. The intra- and inter-day accuracy and imprecision values were 85.9-112.8% and within 13.3%, respectively. The plasma concentrations of edoxaban, 4CA-EDX, and ND-EDX in the patients had ranges of 17.8-102, 1.67-25.7, and 0.685-5.34 ng/mL, respectively. All the analytes were measurable within their calibration curves. In conclusion, this validated method for the simultaneous determination of edoxaban and its major metabolites was successfully applied to plasma specimens obtained from patients with atrial fibrillation.