Background: Polycystic ovary syndrome (PCOS) is one of the most common endocrine disorders among reproductive-age women and is frequently associated with menstrual irregularities. Vitamin D deficiency has emerged as a potential contributing factor to metabolic and reproductive dysfunction in PCOS. Objective: To determine the relationship between vitamin D deficiency and menstrual irregularities among women diagnosed with PCOS. Methodology: A cross-sectional analytical study was conducted at a tertiary care hospital in Lahore, Pakistan, from January 2024 to December 2024. A total of 180 women aged 18–35 years diagnosed with PCOS according to Rotterdam criteria were enrolled. Serum 25-hydroxyvitamin D levels were measured using chemiluminescent immunoassay. Menstrual patterns were recorded and categorized as regular cycles, oligomenorrhea, or amenorrhea. Statistical analysis was performed using SPSS version 26. Correlations between vitamin D status and menstrual abnormalities were assessed using chi-square and logistic regression analyses. Results: The mean age of participants was 26.8±4.3 years. Vitamin D deficiency (<20 ng/mL) was observed in 72.2% of participants. Menstrual irregularities were present in 78.3% of women. Women with vitamin D deficiency had significantly higher rates of oligomenorrhea (52.3%) and amenorrhea (18.5%) compared to those with sufficient vitamin D levels (p<0.001). Serum vitamin D levels showed a significant inverse association with menstrual cycle length (r=-0.41, p<0.001). Conclusion: Vitamin D deficiency is highly prevalent among women with PCOS and is significantly associated with menstrual irregularities. Early detection and correction of vitamin D deficiency may improve reproductive outcomes in affected women.