The present study determined the effect of tagatose supplementation on postprandial hyperglycemia in normal (n = 54) and hyperglycemic subjects [n = 40, impaired fasting glucose (IFG) and newly diagnosed type 2 diabetes]. In a double-blind crossover designed study, study subjects were randomly assigned to consume a sucralose–erythritol drink (the placebo) or a tagatose-containing drink (the test) with a seven-day interval. Finally, 85 subjects completed the study (normal, n = 52; hyperglycemic, n = 33). Blood samples were collected at 0, 30, 60 and 120 min after ingestion and analyzed for fasting and postprandial levels of glucose, insulin and C-peptide. Basic anthropometric parameters and lipid files were also measured. Hyperglycemic subjects were basically older and heavier, and showed higher levels of triglyceride, total- and LDL-cholesterols and apolipoprotein AI and B compared with normal subjects. After consuming the tagatose (5 g)-containing drink, hyperglycemic subjects had a significant reduction in serum levels of glucose at 120 min (p = 0.019) and glucose area under the curve (AUC) (p = 0.017), however these were not observed in normal subjects. When ages were matched between the two groups, the glucose response patterns were shown to be similar. Additionally, normal subjects who received a high-dose of tagatose-containing drinks (10 g) showed significantly lower levels of insulin at 30 min (p = 0.004) and 60 min (p = 0.011), insulin AUC (p = 0.009), and C-peptide at 30 min (p = 0.004), 60 min (p = 0.011) and C-peptide AUC (p = 0.023). In conclusion, a single dietary supplement in the form of a tagatose-containing drink may be beneficial for controlling postprandial glycemic response in Koreans.