Black soy peptides have been shown to possess properties that may decrease blood pressure (BP). To examine the effects of black soy peptide supplementation on BP and oxidative stress in subjects with prehypertension or stage I hypertension, 100 participants with an initial untreated systolic blood pressure (SBP) of 130-159 mm Hg, diastolic blood pressure (DBP) of 80-99 mm Hg or both were enrolled. Participants were randomly assigned to either a group ingesting supplement containing 4.5 g black soy peptides daily or a placebo group for 8 weeks. SBP and DBP decreased after 8-week black soy peptide supplementation versus controls (P<0.001). At 8 weeks, SBP decrease was significantly greater for the black soy peptide group (-9.69 ± 12.37 mm Hg) than for the control group (-2.91 ± 13.29 mm Hg) after adjusting for the baseline levels (P = 0.015). Plasma malondialdehyde (MDA) and urinary 8-epi-prostaglandin F2α decreased (P = 0.004 and P = 0.046, respectively) and plasma superoxide dismutase (SOD) activity increased (P<0.001) following 8 weeks of black soy peptide supplementation versus baselines. The MDA decreases (P = 0.022) and SOD activity and nitric oxide (NO) increases (P = 0.022 and P<0.001, respectively) were greater for the black soy peptide group than for the control group. Changes in SBP negatively correlated with changes in NO (r = -0.343, P = 0.001). Changes in angiotensin-converting enzyme activity negatively correlated with NO decreases (r = -0.490, P<0.001) and SOD activity increases (r = -0.338, P = 0.001). Black soy peptide dietary supplementation significantly reduces SBP and oxidative stress in patients with prehypertension and stage I hypertension.