Background: IVF/ICSI success depends on the proportion of fertilizable metaphase II (MII) oocytes, yet direct evidence linking mature-oocyte rate to clinical outcomes remains limited. This study aimed to assess the impact of mature oocyte rate on clinical pregnancy outcomes in a large retrospective cohort of women undergoing ICSI. Methods: A total of 2,565 women who underwent ICSI cycles at a single IVF center in Türkiye were retrospectively analyzed. Patients were stratified into three groups according to mature oocyte proportion: Group 1 (0-50%), Group 2 (51-75%), and Group 3 (76-100%). Data were analyzed per initiated ICSI cycle. Embryo transfer was performed on day 3 or day 5 post-fertilization, depending on embryo quality and patient characteristics. Logistic regression analysis was performed to assess the independent effect of mature oocyte rate on pregnancy outcomes. Statistical significance was set at p<0.05. Results: Clinical pregnancy rates were significantly lower in Group 1 (10.23%) compared to Group 2 (28.96%) and Group 3 (29.99%) (p<0.001). Live birth and implantation rates increased in the higher maturity groups (7.5% vs. 21.5% vs. 22.9% and 12.1% vs. 24.3% vs. 26.0%, respectively), whereas miscarriage rates decreased (18.9% vs. 13.4% vs. 12.8%). Logistic regression analysis confirmed that a higher mature oocyte proportion was an independent predictor of clinical pregnancy (Group 2: OR=3.4, 95%CI: 2.5-4.6; Group 3: OR=3.5, 95%CI: 2.6-4.7; p<0.001). Conclusion: This large-scale retrospective cohort analysis demonstrates that mature oocyte proportion is an important prognostic factor of IVF success. Mature oocyte proportion should be considered an essential parameter in clinical practice and patient counseling.