A reliable and sensitive liquid chromatography-tandem mass spectrometry assay has been proposed for the selective determination of diflunisal in the presence of its glucuronide metabolites. The analyte and clofibric acid as internal standard (IS) are extracted from 50 µL of human plasma by solid-phase extraction. Chromatographic separation is conducted on a Prodigy ODS 3V column (150 × 4.6 mm, 5 µm) under isocratic conditions. The possible interference of acyl glucuronide and phenolic glucuronide, the two major inactive metabolites of diflunisal, is also checked in plasma samples. Detection of the analyte and IS is achieved by tandem mass spectrometry, operating in negative ionization and multiple reaction monitoring acquisition mode. The limits of detection and quantitation of the method are 0.10 and 1.00 µg/mL, respectively, with a linear dynamic range of 1.00-160 µg/mL for diflunisal. The intra-batch and inter-batch precision (percent coefficient of variation) is ≤4.2% and the mean recovery is >92% for diflunisal across quality control levels. The method is successfully applied to a bioequivalence study of a 500 mg diflunisal tablet formulation in 30 healthy Indian male subjects under fasting conditions. The reproducibility in the measurement of study data is demonstrated by the reanalysis of 120 incurred samples.