Primary dysmenorrhea is a common gynecological disorder in women of reproductive age. Despite the effective conventional treatments such as nonsteroidal anti-inflammatory drugs and oral contraceptives, researchers have always been looking for alternative drugs due to the adverse effects and limited efficacy of these medications. Glycyrrhiza glabra L. (G. glabra), commonly known as Licorice, has been applied for a long time as a plant with multiple therapeutic potencies in Traditional Persian Medicine (TPM). This study was designed to evaluate the effect of the G. glabra on primary dysmenorrhea. Sixty patients with moderate and severe dysmenorrhea were randomly divided into two groups; one group received 400 mg Ibuprofen tablets every 8 h and placebo syrup and the other received 5 cc of G. glabra syrup two times a day and placebo tablets. The patients took the drugs from the first day of menstruation to fifth for two consequent cycles. The primary pain intensity and its changes were evaluated in each group and compared between two groups. The reduction of pain intensity was 5.85 (±3.11) in the G. glabra group compared with 6.92 (±1.87) in the Ibuprofen group (p < 0.001). No significant difference detected between the two groups (p = 0.151). No serious side effects were reported during the study. This study suggests that we can use G. glabra to relieve pain in the patients with primary dysmenorrhea; although studies with a larger sample size may lead to more comprehensive perceptions about the efficacy of G. glabra.