INTRODUCTION: Adolescence is a critical period for health, as conditions like polycystic ovarian syndrome (PCOS) can affect long-term outcomes, including diabetes and other non-communicable diseases in adulthood. This study evaluated the effects of Nigella sativa L. extract on glycemia among adolescents with PCOS. MATERIALS AND METHODS: This secondary analysis used data from a randomized controlled trial conducted between March 2022 and March 2023. One hundred sixteen adolescent girls aged 12-18 years with PCOS were randomized into two groups. The intervention group received 1000 mg/day of Nigella sativa extract for 16 weeks, while the control group received 10 mg/day of medroxyprogesterone for 10 days per menstrual cycle over the same period. Fasting plasma glucose (FPG) and one- and two-hour post-prandial glucose levels were measured at baseline and after the intervention. RESULTS: 103 completed the study (50 in the Nigella sativa group and 53 in the control group). At baseline, there were no significant differences in FPG (p = 0.294), though the control group had higher one-hour (p = 0.002) and two-hour (p = 0.006) post-prandial glucose levels. Post-intervention, significant interaction effects were observed for FPG (p = 0.004) and two-hour post-prandial glucose (p = 0.023), indicating more significant reductions in the Nigella sativa group compared to the control group. CONCLUSIONS: Considering the observed effect of Nigella sativa supplementation on FPG and two-hour post-prandial glucose, it may offer a complementary approach to managing glycemia in adolescent PCOS. However, further research is warranted.