We investigated the effect of Panax ginseng extract, which is rich in the ginsenoside protopanaxatriol (Ginseol K-g1), on blood pressure (BP). Adults over 20 years old with a systolic BP (SBP) between 120 and 159 mm Hg or a diastolic BP (DBP) between 80 and 99 mm Hg were included. At the end of an initial 2-week washout period, the patients were divided into three groups: the control group (placebo), the low-dose Ginseol K-g1 group (100 mg), and the high-dose Ginseol K-g1 (300 mg) group. The primary end point was the difference in seated SBP (seSBP) and seated DBP (seDBP) changes between the placebo and Ginseol K-g1 groups after 8 weeks of treatment. A total of 90 subjects participated in the study (mean age; 55.2 ± 11.8 years, 43 males). At week 8, levels of seSBP and seDBP were significantly decreased from baseline in the high-dose Ginseol K-g1 group (-3.1 mm Hg and -2.3 mm Hg, respectively, p < 0.05). In contrast, there was no significant decrease in seSBP or seDBP in the control or low-dose Ginseol K-g1 groups. No significant difference of seSBP and seDBP was identified among the three treatment groups at week 8. In patients who had a seSBP ≥ 130 mm Hg or an seDBP ≥ 85 mm Hg, the high dose of Ginseol K-g1 decreased the BP compared with the control group at week 4; however, there was no significant difference at week 8. The proportions of patients who experienced adverse events were comparable among the treatment groups. In conclusion, Ginseol K-g1 has a favorable effect on BP after 4 weeks of treatment, especially at a high dose. However, the effect is not maintained over 8 weeks. (Clinical trial registration information is available at http://www.clinicaltrials.gov , identifier: NCT01483430.).