Aims: The present study was conducted to evaluate the safety of celery seed extract (Apium graveolens), as a medicinal herb with active ingredients such as 3-n-butylphthalide (NBP), in hypertensive patients. Methods: This study was a randomized, triple-blind, placebo-controlled, cross-over clinical trial. Hypertensive patients (51 participants) received 4 celery seed capsules (a total of 1.34g extract per day) or 4 placebo capsules per day for 4-weeks as a supplement to their usual medication regimen. Results: The results indicated that the celery seed capsule not only was safe for hypertensive patients but caused a reduction in BP, FBS, and lipid profile values. Also, it had beneficial effects on kidney and liver functions. No significant change was observed in blood cells and serum electrolytes (P>0.05). The mean reduction in BUN and SCr were 3.43 and 0.075 mg/dL, and in SGPT and SGOT were 4.08 and 3.03 U/L, respectively (P<0.05). FBS reduced from 108.53 to 97.96 mg/dL after 4-weeks of celery administration (P<0.01). The decrease in TC, TG, LDL, and increase in HDL were 16.37, 16.22, 11.84, and 2.52 mg/dL, respectively (P<0.001). Conclusions: According to the promising results of this clinical trial, celery seed extract can be considered a safe supplement for hypertensive patients.