The present study aimed to determine the effect of black soy peptide supplementation on glucose control in subjects with prediabetes (impaired fasting glucose or impaired glucose tolerance) and newly diagnosed type 2 diabetes mellitus (DM). In this double-blind, placebo-controlled study, subjects with prediabetes and type 2 DM were randomly assigned to the placebo control group or the black soy peptide intervention group. We determined fasting serum concentrations of glucose, hemoglobin A1c, insulin, and free fatty acids, performed a 2-hour postload glucose (2-hour PG) test, and compared serum lipid profiles before and after the 12-week supplementation. In particular, subjects with fasting glucose ≥ 110 mg/dL who consumed black soy peptides tended to have lower fasting glucose levels (two-tailed test, P = .098; one-tailed test, P = .049) and had a significant reduction in 2-hour PG level (two-tailed P = .012, one-tailed P = .006), compared with baseline levels. The changes in 2-hour PG levels were also statistically significant in the intervention group (-41.25 ± 13.67 mg/dL) compared with the placebo group (12.42 ± 9.80 mg/dL; two-tailed P = .015, one-tailed P = .008). In contrast, hemoglobin A1c levels were not significantly improved by the dietary intervention. In conclusion, black soy peptide supplementation may be beneficial for controlling fasting blood glucose levels and 2-hour PG levels.