This study aimed to investigate the correlation between serum vitamin D levels and asthma severity in children aged 5-15 years. Methodology: A cross-sectional study was conducted from February 2, 2022, to July 30, 2022 at JPMC Karachi, involving 150 children diagnosed with asthma at a tertiary care hospital. Participants were selected based on strict inclusion and exclusion criteria, ensuring a focus on asthma without confounding chronic conditions. Serum 25-hydroxyvitamin D [25(OH)D] levels were measured, and asthma severity was classified according to the Global Initiative for Asthma (GINA) guidelines. Data were collected through blood samples, spirometry, and structured questionnaires. Statistical analysis included Pearson’s correlation to evaluate the relationship between vitamin D levels and asthma severity, ANOVA to compare vitamin D levels across severity categories, and multivariate regression analysis to control for potential confounders such as age, gender, BMI, and sunlight exposure. Results: The study found a significant negative correlation between serum vitamin D levels and asthma severity (r = -0.48, p < 0.001). Children with severe asthma had the lowest mean vitamin D levels (19.5 ± 6.3 ng/mL), while those with mild asthma had the highest levels (28.7 ± 7.8 ng/mL) (p = 0.001). Multivariate regression analysis confirmed the association between lower vitamin D levels and increased asthma severity, independent of other variables (β = -0.34, p = 0.002). Conclusion: Lower serum vitamin D levels are significantly associated with increased asthma severity in children. Addressing vitamin D deficiency may play a crucial role in managing asthma severity in pediatric populations.