Background: Premenstrual Syndrome (PMS) is a common problem in women. Nigella sativa has been suggested for its anti-inflammation and analgesic effects. This study was conducted to evaluate the effect of Nigella sativa oil on PMS. Methods: This double-blind clinical trial was conducted on 124 female students within the age range of 18-25 years living in the dormitories of Sabzevar University of Medical Sciences. Participants were randomly divided into two groups. The intervention group (IG) rubbed 1-2 drops of Nigella sativa oil on their fontanels at night for sev­en days before their three menstrual cycles. The placebo group (PG) rubbed placebo in the same way. After three cycles, pain severity was measured by the visual analog scale. Data analysis was carried out using the Mann-Whitney U test and analysis of covariance. Results: The mean age of participants, the mean age of menarche, and the mean age of PMS onset were 20.55 ± 0.2, 13.52 ± 0.15, and 15.35 ± 0.3 years old, respectively. The results showed that Nigella sativa oil reduced the severity of all PMS symptoms except in terms of depression and abdominal bloating in IG compare to the PG. Conclusion: Nigella sativa oil in women with premenstrual syndrome can be a promising, safe, and easily available analgesic supplement.