Background: Vitamin D plays a crucial role in various physiological processes, including mental health. Recent studies suggest a potential link between vitamin D deficiency and depression, yet the association remains inconclusive. This study aims to evaluate the relationship between serum vitamin D levels and depression among adults. Materials and Methods: A cross-sectional study was conducted among 300 adults aged 18–60 years, recruited from a tertiary care hospital. Participants were assessed for depression using the Patient Health Questionnaire-9 (PHQ-9). Serum 25-hydroxyvitamin D [25(OH)D] levels were measured using chemiluminescent immunoassay. Based on vitamin D levels, participants were categorized into three groups: deficient (30 ng/mL). Statistical analysis was performed using SPSS, with a significance level set at p< 0.05. Results: The mean serum vitamin D level was 18.4 ± 5.2 ng/mL. Among the participants, 55% were vitamin D deficient, 30% had insufficient levels, and 15% had sufficient levels. Depression was significantly more prevalent among those with vitamin D deficiency (72%) compared to those with insufficient (50%) and sufficient levels (25%) (p< 0.001). A strong negative correlation was observed between vitamin D levels and PHQ-9 scores (r = -0.62, p< 0.001). Logistic regression analysis revealed that individuals with vitamin D deficiency had a 3.5-fold higher risk of depression compared to those with sufficient levels. Conclusion: This study highlights a significant association between vitamin D deficiency and depression in adults. Given the high prevalence of vitamin D deficiency, screening and supplementation may serve as potential strategies to mitigate depressive symptoms. Further longitudinal studies are needed to establish a causal relationship.