Anthocyanins are major color pigments in plants. Their biosynthetic pathways are well established, and the majority of these biosynthetic enzymes have been identified in model plants such as Arabidopsis, maize, and petunia. One exception in Arabidopsis is UDP-glucose:flavonoid 3-O-glucosyltransferase (UF3GT). This enzyme is known as Bronze-1 (Bz1) in maize, where it converts anthocyanidins to anthocyanins. Phylogenetic sequence analysis of the Arabidopsis thaliana UDP-glycosyltransferase (UGT) family previously indicated that UGT78D1, UGT78D2, and UGT78D3 cluster together with UF3GTs from other species. Here, we report that UGT78D2 encodes a cytosolic UGT that is functionally consistent with maize Bz-1. Biochemically, UGT78D2 catalyzes the glucosylation of both flavonols and anthocyanidins at the 3-OH position. A T-DNA-inserted ugt78d2 mutant accumulates very little anthocyanin and lacks 3-O-glucosylated quercetin. Expression analysis indicated that UGT78D2, in opposite to BANYULS, is highly expressed in anthocyanin-accumulating seedlings but repressed in condensed tannin-accumulating seed coats. This suggests that the reciprocal regulation of these two genes is important in directing the metabolic flux to either anthocyanins or condensed tannins. Consistent with this, the ectopic expression of UGT78D2 produces purple-colored seed coats due to the accumulation of anthocyanins. Taken together, our data indicate that UGT78D2 encodes an enzyme equivalent to maize Bz1, and that the reciprocal regulation of UGT78D2 and BANYULS is critical for the regulation of metabolic flux of anthocyanidins in Arabidopsis.