The objective of this study was to examine the effects of Morinda citrifolia (noni) extract and its constituents on α-melanocyte stimulating hormone (α-MSH)-stimulated melanogenesis in cultured murine B16 melanoma cells (B16 cells). A 50% ethanolic extract of noni seeds (MCS-ext) showed significant inhibition of melanogenesis with no effect on cell proliferation. MCS-ext was more active than noni leaf and fruit flesh extracts. Activity guided fractionation of MCS-ext led to the isolation of two lignans, 3,3'-bisdemethylpinoresinol (1) and americanin A (2), as active constituents. To elucidate the mechanism of melanogenesis inhibition by the lignans, α-MSH-stimulated B16 cells were treated with 1 (5 μM) and 2 (200 μM). Time-dependent increases of intracellular melanin content and tyrosinase activity, during 24 to 72 h, were inhibited significantly by treatment with the lignans. The activity of 1 was greater than that of 2. Western blot analysis suggested that the lignans inhibited melanogenesis by down regulation of the levels of phosphorylation of p38 mitogen-activated protein kinase, resulting in suppression of tyrosinase expression.