Anthocyanidin synthase (ANS), a 2-oxoglutarate (2OG) iron-dependent oxygenase, catalyzes the penultimate step in the biosynthesis of anthocyanin. This reaction is responsible for the formation of the colored anthocyanidins from the colorless leucoanthocyanidins. A full-length cDNA was isolated from purple-fleshed sweet potato (Ipomoea batatas (L.) Lam) cv. Yamakawamurasaki, designated IbANS, containing a 1,086-bp open reading frame encoding a 362-amino-acid polypeptide. Multiple alignments revealed that the deduced IbANS protein had high identity to ANS proteins of other plants such as Ipomoea nil (90.8% identities), Ipomoea purpurea (91.4% identities), and Brassica juncea (72.7% identities). Structural analysis showed that the IbANS protein might belong to the 2OG and Fe(II)-dependent oxygenase, containing three binding sites of 2OG (H236, D238, and H292) and three binding sites of Fe(II) (Y221, R302, and S304). Phylogenetic tree analysis revealed that IbANS shared the close relationships with I. nil and I. purpurea. Southern blotting showed that there were two copies of the IbANS gene in this genome. Real-time quantitative polymerase chain reaction revealed that expression of the IbANS gene was highest in storage roots and lowest in leaves. IbANS was expressed most abundantly during the formation of storage roots. In five cultivars of sweet potato, IbANS expression was strongly associated with anthocyanin accumulation, suggesting that ANS gene expression was associated with activation of anthocyanin biosynthesis.