Aim: This clinical trial investigated the effect of fennel on reducing the severity of primary dysmenorrhea in women. Methods: This study was a clinical trial with a triple-blind control group. The number of samples was 63 women aged 15-25 years. They were selected from the Farhangian Clinic using a convenience sampling method. To measure the severity of primary dysmenorrhea, the visual analog scale (VAS) was used. The clients were screened by a gynecologist and obstetrician. Participants were evaluated at baseline, cycle 1, and cycle 2 in fennel and placebo groups. Data were analyzed using the MANOVA test in SPSS26 software. Results: The results revealed that the mean severity of pain in the experimental and placebo groups at baseline did not differ from each other (P>0.05). However, the results showed that the severity of pain in the fennel group (F=6.430, P<0.05) was reduced compared to the placebo group in cycle 1. The results also revealed that the severity of pain in the fennel group (F=5.548, P<0.05) was reduced compared to the placebo group in cycle 2. Discussion: The results revealed that fennel can reduce the severity of pain of primary dysmenorrhea compared to placebo. Thus, health professionals are recommended to use fennel to reduce the severity of primary dysmenorrhea due to its low side effects and high impact on reducing primary dysmenorrhea.