Persimmon is a fruit rich in polyphenols (proanthocyanidins or condensed tannins). Using rats and humans, the effects of Kaki-tannin (Nara-type), persimmon polyphenols prepared using a new method, on postprandial plasma glucose levels were investigated in this study. Kaki-tannin (Nara-type) comprised mainly proanthocyanidins, composed of epicatechin : epicatechin gallate : epigallocatechin : epigallocatechin gallate in a ratio of 1 : 1 : 2 : 2 with a molecular weight of approximately 8,000 Da, with epicatechin gallate as a terminal unit. These polyphenols inhibited amylolytic enzymes, such as α-amylase, maltase, sucrase, and α-glucosidase in vitro, and sodium-dependent glucose transporter 1 in Caco-2 cells. These results suggested that the polyphenols suppressed digestion and absorption in the intestinal tract. The ingestion of 250 mg/kg body weight of the polyphenols significantly suppressed increased blood glucose levels after carbohydrate (2 g/kg body weight of glucose or maltose) loading in rats. In a human trial, 1.88 g of Kaki-tannin (Nara-type) significantly delayed increased plasma glucose levels after carbohydrate (150 kcal of maltooligosaccharides) loading. Thus, Kaki-tannin (Nara-type) holds promise to be developed as a food material that potentially improve blood glucose elevation after meals.