Background & aim: Postpartum blues is the most common postpartum mood disorder, which can turn into a chronic depression. Therefore, the prevention of this disorder is effective for the improvement of family mental health. The present study was conducted to investigate the effect of Melissa officinalis (Lemon balm) on the incidence of postpartum blues. Methods: The present triple blind placebo-controlled clinical trial was performed on 60 women with cesarean section (C-section) at teaching hospitals affiliated to Mashhad University of Medical Sciences, Mashhad, Iran, from 2016 to 2017. The participants were randomly divided into intervention and placebo groups. The interventiongroup was administered lemon balm capsules (500 mg) from the first day after cesarean section three times a day for 10 days. Edinburgh postnatal depression scale was completed by all subjects at three stages, namely on the 3rd-5th, 10th, and 14th days after C-section. The data were analyzed by SPSS software (version 16) using independent t-test, as well as Mann-Whitney U, Chi-square, Fisher's exact tests and Friedman test. Results: The Fisher's exact test showed that the incidence of postpartum blues on the 3rd-5th, 10th, and 14th days after C-section were 3. 3% and 43. 3% (P<0. 00), 3. 3% and 50% (P<0. 00), and 0% and 33. 3% (P<0. 00) in the intervention and placebo groups, respectively. The results of the two-way ANOVA test revealed that the depression score increased with maternal age due to the intervention (P=0. 046). Conclusion: The use of lemon balm could reduce the incidence of postpartum blues, which is one of the most common postpartum psychiatric disorders, without occurance of possible side effects. Therefore, it is recommended to be used, especially in susceptible women for postpartum blues after delivery.