Background: High blood pressure (BP) is a common chronic disease needs long life drug consumption to control. Spironolactone could be used as the fourth-line therapy in patients with resistant hypertension. However, there is no study to determine the effects of low dose spironolactone as a first line therapy in treatment of essential hypertension. The aim of this study is to investigate the effect of low dose spironolactone monotherapy in management of essential hypertension. Methods: In this double blind randomized clinical trial, 40 patients who had stage I essential hypertensionwere randomly divided into two groups: intervention group received spironolactone 25 milligram once daily for one month and control group received placebo once daily. At the baseline and after one month, 24-hour BP holtermonitoring and serum potassium assay were done. Results: Systolic BP was reduced from 143.5  8.2 mmHg to 137.10  7.57 mmHg in the intervention group, while it did not change significantly in control (between group treatment difference = -4.5mmHg, p = 0.004). There was no significant reduction of diastolic BP in the intervention group in comparison to placebo group (between group treatment difference = -1.3 mmHg, p = 0.099). Conclusions: Short course monotherapy with low dose spironolactone is effective in reducing systolic BP in patients with stage I essential hypertension.