Scytonemin is a 544-Da hydrophobic pigment that can absorb UV-A radiation. It is present in cyanobacterial sheaths and is thought to function as a UV protectant. In this study, scytonemin was purified from the terrestrial cyanobacterium Nostoc commune, and its radical-scavenging activity was characterized. The purified scytonemin quenched an organic radical in vitro and accounted for up to 10% of the total activity of an ethanol extract of N. commune. These results suggest that the extracellular UV-absorbing pigment scytonemin has multiple roles, functioning as a UV sunscreen and an antioxidant relevant to anhydrobiosis in N. commune.