A tyrosinase inhibitor was isolated from the whole plant of Barbarea orthocerus Led. (Brassicaceae) by activity-guided fractionation, and identified as (R)-5-phenyl-2-oxazolidinethione (barbarin) by structural analysis followed by comparison with reported spectral data. The compound exhibited significant inhibitory effects on mushroom and murine tyrosinases at more than 1.6 × 10-5 M. Barbarin exhibited IC50 values of 4.2 × 10-5 M on mushroom tyrosinase and of 4.8 × 10-5 M on murine tyrosinase. Kojic acid as a positive control exhibited IC50 values of 3.4 × 10-5 M and 6.0 × 10 -5 M on mushroom and murine tyrosinases, respectively. Therefore, barbarin exhibited a similar level of inhibitory potency with kojic acid used as a positive control. In a kinetic study with various concentrations of L-dopa as the substrate, barbarin was identified as an uncompetitive inhibitor and kojic acid as a mixed inhibitor of both mushroom and murine tyrosinases. Barbarin exhibited KEIS values of 3.3 × 10-5 M and 3.6 × 10-5 M on mushroom and murine tyrosinases, respectively. Kojic acid exhibited KEIS and KEI values of 2.4 × 10-5 M and 2.2 × 10-5 M on mushroom tyrosinase and those of 8.9 × 10-5 M and 7.2 × 10-5 M on murine tyrosinase, respectively.