Background:Several studies are conducted on Premenstrual Syndrome (PMS). However, a few herbal surveys exist on the treatment of PMS in Iran. Due to the sedative effects of Melissa officinalis (M. officinalis), this question comes to mind that “can it be used in the treatment of PMS symptoms?”Objectives:The current study aimed to assess the effect of M. officinalis capsule on the intensity of PMS in high-school girls.Materials and Methods:A double-blind randomized, placebo-controlled trial was performed on 100 high school girls from 2013 to 2014. The intervention group (n = 50) received 1200 mg of M. officinalis essence daily from the first to the last day of their menstrual cycle for three consecutive cycles. The second group (n = 50) received the placebo. The premenstrual symptoms screening tool was used to assess the intensity of PMS symptoms in the two groups before and one, two, and three months after the intervention. The data were analyzed using paired t-test and repeated measures analysis of variance.Results:The results of repeated measures test revealed a significant reduction (P < 0.001) in PMS symptoms. Overall, the mean score of PMS intensity in the intervention group was 42.56 + 15.73 before the intervention and changed to 32.72 ± 13.24, 30.02 ± 12.08, and 13.90 ± 10.22 at the three consecutive months after the intervention, respectively (P = 0.001).Conclusions:M. officinalis capsules were effective in reduction of the PMS symptoms. Yet, application of this medication requires further investigations.