Background <p>Premature Ovarian Insufficiency (POI) is a condition characterized by the inability to ovulate before the age of 40. The occult type of POI (OPOI) is associated with a significant decrease in fertility and normal FSH levels. Fennel (Foeniculum vulgare Mill.) is known for its antioxidant properties and potential benefits on ovarian function. This study aimed to determine the effect of fennel extract on Anti-Müllerian Hormone (AMH) levels and antral follicular count (AFC) in infertile women with occult premature ovarian insufficiency.</p> Methods <p>This randomized controlled clinical trial was conducted at Tabriz University of Medical Sciences. 60 Infertile women with OPOI were randomly assigned to either the intervention group, who received fennel extract, or the control group, who received a placebo, with a ratio of 1:1. AMH measurement and AFC were performed at baseline and after 60 days, on the third day of menstruation.</p> Results <p>AMH level and AFC at the baseline were not statistically significant between fennel and the placebo groups(<i>p</i> &gt; 0.05). After the intervention, the difference in changes of AMH between the two groups was not statistically significant (<i>p</i> = 0.986). However, AFC in fennel group was significantly higher than the placebo group after intervention (mean difference = 1.6; confidence interval of 95%: from 0.07 to 3.1; <i>p</i> = 0.040).</p> Conclusion <p>Fennel extract may have a beneficial effect on AFC in infertile women with occult POI, but had no effect on serum AMH levels. Further studies are needed to reach more definitive results in this field.</p> Trial registration <p>This trial has been registered at Iranian Registry of Clinical Trials (IRCT20111219008459N15). Submitted 1 February 2023, registered 5 February 2023 prospectively (<a href="https://irct.behdasht.gov.ir/user/trial/67583/view">https://irct.behdasht.gov.ir/user/trial/67583/view</a>).</p>