During the course of screens to identify anti-melanogenic agents from natural resources, we found that the methanol extract of the dried flower of Inula britannica L. inhibited melanin synthesis in cultured melanoma cells stimulated with 3-isobutyl-1-methylxanthine (IBMX). A bioassay-guided isolation of the chloroform fraction of the I. britannica using an in vitro melanogenesis inhibition assay led to the isolation of sesquiterpenes, 1-O-acetylbritannilactone (1), britannilactone (2) and neobritannilactone B (3). Compounds 1 and 2 significantly reduced melanin production in a dose-dependent manner with IC50 values of 13.3 and 15.5 μM, respectively, whereas compound 3 was found to be cytotoxic. Compound 1 also inhibited the tyrosinase activity only in cell based-systems. Western blot analysis indicated that compound 1 inhibited melanogenesis by activating extracellular signal-regulated kinase (ERK) and Akt signaling and also inhibiting cAMP related binding protein, which regulates its downstream pathway, including tyrosinase, tyrosinase related protein-1 and TRP-2. These results demonstrated that compound 1, a major sesquiterpene from the flowers of I. britannica, exhibited anti-melanogenic activity by suppression of tyrosinase expression via ERK and Akt signaling. Taken together, our results suggest that these compounds may act as potent natural skin-lightening agents.