This study was conducted to identify the anti-melanogenesis constituents from a seaweed Dictyota coriacea (Holmes). Three known compounds, viz. 1,9-dihydroxycrenulide (1), epiloliolide (2) and D-mannitol (3), were isolated from the ethanol extract. The melanin synthesis inhibition activities were evaluated using B16F10 melanoma cells for the isolates. Compared with the positive control, arbutin, compounds 1 and 2 exhibited more potency, showing 27.8 and 22.6% inhibition activities at a substrate concentration of 30 microg/mL. Our studies also indicate that these compounds are not cytotoxic. Hence, they might prove to be useful therapeutic agents for treating hyperpigmentation and effective components of whitening cosmetics.