The anti-hypertensive effect of GABA-rich Chlorella was studied after oral administration for 12 weeks in the subjects with high-normal blood pressure and borderline hypertension in the placebo-controlled, double-blind manner in order to investigate if GABA-rich Chlorella, a dietary supplement, is useful in control of blood pressure. Eighty subjects with Systolic blood pressure (SBP) 130-159 mmHg or diastolic blood pressure (DBP) 85-99 mmHg (40 subjects/group) took the blinded substance of GABA-rich Chlorella (20 mg as gamma-aminobutyric acid) or placebo twice daily for 12 weeks, and had follow-up observation for an additional 4 weeks. Systolic blood pressure in the subjects given GABA-rich Chlorella significantly decreased compared with placebo (p < 0.01). Diastolic blood pressure had the tendency to decrease after intake of GABA-rich Chlorella. Neither adverse events nor abnormal laboratory findings were reported throughout the study period. Reduction of SBP in the subjects with borderline hypertension was higher than those in the subjects with high-normal blood pressure. These results suggest that GABA-rich Chlorella significantly decreased high-normal blood pressure and borderline hypertension, and is a beneficial dietary supplement for prevention of the development of hypertension.