The effects of 12 weeks of supplementation with a dieckol-rich extract (AG-dieckol) from brown algae, Ecklonia cava, on glycemic parameters, serum biochemistry, and hematology were investigated in this study. Eighty pre-diabetic male and female adults were enrolled in a randomized, double-blind, placebo-controlled trial with parallel-group design. Subjects were randomly allocated into two groups designated as placebo and AG-dieckol (1500 mg per day). Compared with the placebo group, the AG-dieckol group showed a significant decrease in postprandial glucose levels after 12 weeks. The AG-dieckol group also showed a significant decrease in insulin and C-peptide levels after 12 weeks, but there was no significant difference between the AG-dieckol and placebo groups. There were no significant adverse events related to the consumption of AG-dieckol, and biochemical and hematological parameters were maintained within the normal range during the intervention period. In conclusion, these results demonstrate that AG-dieckol supplementation significantly contributes to lowering postprandial hyperglycemia and in reducing insulin resistance. Furthermore, we believe that based on these results the consumption of phlorotannin-rich foods such as marine algae may be useful for the treatment of diabetes.