Background: Vitamin D supports oocyte development and ovarian steroidogenesis; deficiency is linked to preeclampsia and poor oocyte quality. The purpose of the current study was to investigate the relationship between vitamin D levels and oocyte quality in women undergoing ICSI cycles. Methods: In this case-control study, 252 women were identified from medical records and classified into high-quality (n=126) and low-quality oocyte groups (n=126). Serum vitamin D levels and anti-Müllerian hormone (AMH) were recorded. Oocyte quality was assessed by an embryologist using predefined criteria for morphology and maturity. Statistical analysis included Mann–Whitney tests and multivariable logistic regression (p<0.05). ROC analysis was applied to determine the optimal vitamin D cutoff, reporting AUC, sensitivity, and specificity and statistical significance was set at p<0.05. Results: Patients in the high-quality oocyte group had a higher median serum vitamin D level compared with those in the low-quality group (25.2 vs. 21.9 ng/ml). Logistic regression showed that sufficient vitamin D levels (≥18 ng/ml) were associated with increased odds of having high-quality oocytes (OR=1.85, 95%CI: 1.07–3.22). AMH levels were also positively associated with oocyte quality (OR=1.42, 95%CI: 1.11–1.82). The ROC analysis yielded an AUC of 0.61 (95%CI: 0.53–0.68), indicating modest discriminatory ability. Conclusion: Serum vitamin D levels are associated with oocyte quality in women undergoing ICSI cycles. While higher vitamin D levels are linked to better oocyte quality, no causal inference can be drawn from this case–control study design, and interventional studies are needed to evaluate whether correction of vitamin D deficiency improves reproductive outcomes.