Two polyphenolic compounds; 1,3,6-tri-O-galloyl-β-D-glucopyranose (TGG) (1) and methyl gallate (2) have been isolated from the aerial parts of Citharexylum quadrangulare Jacq. They were isolated for the first time from the genus Citharexylum and their structures have been established on the basis of spectroscopic methods in addition to comparison with literature data. The stimulatory activity of the isolated compounds was investigated on melanin synthesis where they stimulate the intracellular synthesis of melanin in murine melanoma B16F1 cells in cultured cell lines. The melanin content was dramatically increased by these compounds in a dose dependent manner and at nontoxic concentrations where IC50 of TGG was 20.4 µM and that of methyl gallate was 16.1 µM. Moreover, the isolated compounds were tested for cytotoxic activity where IC50 of compounds (1) & (2) were 1.5 and 10.8 µM, respectively. Also, they exhibited higher radical scavenging activity than ascorbic acid in DPPH assay system with IC50 of 1.5 and 10.8 µM for compounds (1) & (2) respectively. Dramatic stimulatory effect of the isolated compounds on melanogenesis might be applied to various conditions of hypopigmentation-related disorders like Vitiligo as an adjunctive therapy in addition to its safely use as an antioxidant. The obtained results indicated that these active compounds could be used safely in cosmetic purposes.