Introduction Fennel has many medicinal properties and is used in the treatment of dysmenorrhea. Given the widespread use of herbal medicine among women for menstrual problems and considering the fact that there has been no study to date about the effect of fennel on menstrual bleeding and duration of menstrual bleeding through systematic review, the present study was conducted to determine the effect of fennel on the amount (primary outcome) and duration of menstrual bleeding and its side-effects (secondary outcomes). Materials All articles, including Persian and English, with no time limit were searched for in the following databases: Medline (through PubMed), Scopus, EMBASE (through Ovid), Cochrane Library, Web of Sciences, Google Scholar, ProQuest, Clininaltrial.gov, SID, Magiran, Irandoc, and Iranmedex, using MeSH terms, including menstrual bleeding, menstruation, severity of bleeding, hypermenorrhea, menorrhagia, fennel, fennelin, Foeniculum vulgare , dysmenorrhea, and painful menstruation, which were searched separately or in combination. Two authors separately reviewed articles to determine the inclusion criteria, and any disagreement was resolved by reaching consensus with a third person. Results A total of 7993 articles were identified through searching the databases, of which 7327 were excluded as duplicates and 666 were screened for inclusion. Six hundread and forty six were excluded by title and abstract based on not being relevant to the review and being conducted on animals. Eventually, six articles were included in the study and four articles entered into the meta-analysis. The results from meta-analysis showed that using fennel caused a significant increase in mean menstrual bleeding in the first cycle after treatment in the intervention group compared to the control (Std. mean difference: 0.46; 95 % CI: 0.18–0.73; p = 0.001; I 2 = 9 %). However, it had no significant effect on menstrual bleeding in the second cycle after treatment (Mean difference: 1.44; 95 % CI:-5.09 to 7.96; p = 0.67; I 2 = 0 %). Conclusions The results of meta-analysis of four articles showed that in the first cycle after treatment, use of fennel increased menstrual bleeding in the intervention group compared to the control, but meta-analysis of two articles showed no significant difference between intervention and control groups in the amount of menstrual bleeding in the second cycle after treatment. Given the poor quality of articles, conducting clinical trials to determine the effect of fennel on menstrual bleeding appears necessary.