Hypertension (HT) is a lifestyle-related disease and dietary modifications are effective for its management and prevention. We conducted a randomized, double-blind, placebo-controlled trial to evaluate the efficacy of treatment with an oral Nigella sativa (NS) seed extract supplement in patients with mild HT. Subjects were randomized into three groups: a placebo and two test groups that received 100 and 200 mg of NS extract twice a day. After 8 weeks, systolic blood pressure (SBP) values in both case groups were found to be significantly reduced when compared with the baseline values for each group. In addition, the decrease in SBP in the two case groups was statistically significant relative to the placebo group (P < 0.05-0.01). Meanwhile, diastolic blood pressure (DBP) values in the case groups were found to be significantly reduced from the baseline and a significant reduction was also observed in these groups (P < 0.01) when compared with the placebo group. In addition, extract administration reduced both SBP and DBP in a dose-dependent manner. Meanwhile, NS extract caused a significant decline in the level of total and low-density-lipoprotein (LDL)-cholesterol relative to baseline data. No complications caused by NS were observed. The results suggest that the daily use of NS seed extract for 2 months may have a blood pressure-lowering effect in patients with mild HT.