Objective: One of the important and common problems of mothers after giving birth is insufficient milk production. To increase it, due to the side effects of chemical drugs that increase milk, medicinal plants have become more popular. In recent years, with the clarification of the need for evidence-based clinical practice, quality and appropriate information is needed. The present study aims to review the results of clinical trials on the effect of milk thistle in increasing breast milk volume. Methods: In this systematic review study, the search for the effects of milk thistle in increasing breast milk volume was conducted in Magiran, MedLib, SID, and IranMedex using the keywords milk thistle, silymarin, milky plants. The search for clinical trials was conducted in EMBASE, PubMed, Scopus, Ovid, and ProQuest using the keywords milk thistle, Silybum marianum, Silymarin Silybin, breast milk production, herbal galactagogues, increase breast milk, and Clinical trial. The publication year was 2005-2020. The criteria for excluding articles were: Unavailability of full texts, irrelevant results, pilot studies, case reports, review studies, and animal studies. Data analysis was done qualitatively. Results: Initial search yielded 624 records. Of these, 4 eligible articles with a sample size of 294 were selected and reviewed. All studies showed that milk thistle plant had a positive effect on increasing milk volume. In these studies, no serious side effects from the consumption of this plant were reported. Conclusion: milk thistle plant increases milk production in new mothers. The number of studies in this field is low, especially in Iran, which has a long history in traditional medicine and use of herbal medicine. More studies are needed in this field.