The methanolic extract of Lawsonia inermis L. (henna) showed significant inhibitory activity toward melanogenesis in B16 melanoma 4A5 cells. Among the constituents isolated from the methanolic extract, luteolin, quercetin, and (±)-eriodictyol showed stronger inhibitory activity than the reference compound, arbutin. Several structure-activity relationships of the flavonoids were suggested, and OGlc<H=OH at the 3-position, OGlc<OH at the 4'-position, and the double bond between the 2- and 3-positions were important. The active constituents suppressed tyrosinase, tyrosinase related protein (TRP)-1, and TRP-2 mRNA expression. The suppression was considered as one of the mechanisms of action. Furthermore, the methanolic extract and several constituents, including luteoloside and spiraeoside, showed anti-plasmin activity, which is considered to play a key role in UV-stimulated melanogenesis in human skin.