Simvastatin, a pro-drug lactone whose major hydroxy acid metabolite, simvastatin acid, inhibits the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase, combined with extended-release (ER) niacin decreases LDL cholesterol and increases HDL cholesterol to a greater extent than either treatment alone. This study developed a highly sensitive LC-MS/MS method for the simultaneous quantification of SV and SVA in human plasma. The plasma samples were pretreated by liquid–liquid extraction with ethyl ether/dichloromethane (3:2, v/v). Chromatographic analysis was conducted using isocratic elution on a Zorbax Eclipse XDB-C18 column (4.6 mm × 150 mm, 5 μm) with a mobile phase of methanol/10 mM ammonium acetate/formic acid (90:10:0.02, v/v/v). The flow rate was 1.0 mL/min (split 0.5:0.5). An API 4000 triple quadrupole mass spectrometer was used for detection with an electrospray ionization interface. The ion transitions used were m/z 435.4→319.0 for SVA in the negative ion mode and 419.1→199.1 for SV in the positive mode. This method exhibited linearity within the concentration range of 0.05 to 15 ng/mL for SV and SVA. The LLOQ was 0.05 ng/mL with acceptable precision and accuracy. The intra- and inter-day precisions were all less than 14.7% (RSD, %). The accuracies were within ±1.85% (RE, %). Compared with earlier reports, this validated method offered a higher sensitivity and smaller cost for the quantification of SV and its metabolite SVA in human plasma. It was successfully applied for the evaluation of a pharmacokinetic study using a fixed dose of a simvastatin and ER niacin combination tablet (10 mg/500 mg) in healthy Chinese volunteers.