Curcumin, a principal curcuminoid of turmeric, gained a lot of attention recently due to its wide spectrum of pharmacological activities in prevention and treatment of various human conditions. The current clinical study is focused on the determination of efficacy and tolerability of curcumin and vitamin D3 combination in patients with early-stage chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL). To support this work, an LC–MS/MS method has been developed for simultaneous determination of curcumin and its metabolites, curcumin glucuronide (COG) and curcumin sulfate (COS), in human plasma using curcumin-d6 as an internal standard for curcumin and BPAG-d6 as an internal standard for COG and COS. In this method, plasma samples were prepared by methanol protein precipitation, and the analytes were separated on a Waters XTerra® MS C18 column (2.1 mm × 50 mm, 3.5 μm) using gradient elution with methanol and 10.0 mM ammonium formate (pH 3.0) at a flow rate of 0.250 mL min−1. Quantitation of curcumin, COG, and COS was carried out by tandem mass spectrometry using negative electrospray ionization in multiple-reaction-monitoring (MRM) mode. The linear calibration range for the method was 2.50–500 ng mL−1 for curcumin, COG, and COS. The method has been validated in accordance with the US-FDA guidelines for bioanalytical method validation. The developed method has been used for the measurement of curcumin, COG and COS concentrations in human plasma samples from a phase II clinical trial.