Camu-camu (Myrciaria dubia, Myrtaceae) is a tropical fruit, and is known to be rich in vitamin C. The fruit is mainly used in juice or vinegar production, and large amounts of byproduct, composed of seed and peel, is generated as agricultural and industrial waste. Our studies on camu-camu seed and peel revealed that the seed contains large amounts of polyphenols (400 mg/g). Further studies on the seed resulted in the isolation of vescalagin and castalagin, C-glycosidic ellagitannins with a hexahydroxydiphenoyl group and a nonahydroxyterphenoyl group attached to the open chain D-glucose, as the main polyphenolic constituents. The structures of these tannins were characterized by their ¹H- and ¹³C-NMR spectra and mass spectrum. The two C-glycosidic ellagitannins represent about 15% of the polyphenolics in the seed of camu-camu fruit, and are likely responsible for the anti-oxidative activity of the seed extract’s anti-oxidative activity.