Peanut skin (Arachis hypogaea L., Fabaceae) is an abundant source for polyphenols, such as proanthocyanidin oligomers. To determine whether proanthocyanidin has beneficial effects on skin, we tested for inhibitory activity of proanthocyanidins isolated from peanut skin on inflammatory cytokine production and melanin synthesis in cultured cell lines. Administration of peanut skin extract (PSE, 200 µg/mL) decreased melanogenesis in cultured human melanoma HMV-II co-stimulated with phorbol-12-myristate-13-acetate. It also decreased production of inflammatory cytokines (PSE at 100 µg/mL), tumor necrosis factor-α and interleukin-6, in cultured human monocytic THP-1 cells in response to lipopolysaccharide. We isolated ten known proanthocyanidins and one new proanthocyanidin trimer from the PSE. The structure of the new compound (5) was determined by 1D- and 2D-NMR and mass spectrometry analyses, and was determined as epicatechin-(2β→O→7,4β→6)-epicatechin-(4β→6)-epicatechin. The other known proanthocyanidins were identified as proanthocyanidin monomers (1), dimers (6-9), trimers (3-5) and tetramers (2, 10, 11). They showed suppressive activities against melanogenesis and cytokine production at concentrations ranging from 0.1-10 µg/mL. Among the tested compounds, suppressive activities of proanthocyanidin dimers or trimers in two assay systems were stronger than those obtained with monomer or tetramers. These data indicate that proanthocyanidin oligomers from peanut skin have the potential to reduce dermatological conditions such as inflammation and melanogenesis.