Background: Chronic spontaneous urticaria (CSU) is a distressing, mast cell–mediated skin disorder with substantial impact on quality of life. Emerging evidence suggests that vitamin D, a key immunomodulatory hormone, may influence inflammatory and allergic diseases, but its relationship with CSU severity remains incompletely understood. Aim: To evaluate the relationship between the severity of chronic spontaneous urticaria and serum vitamin D levels in adult patients attending a tertiary care hospital. Material and Methods: This hospital-based analytical cross-sectional study included 70 adult patients diagnosed with CSU. Disease severity was assessed using the Urticaria Activity Score over 7 days (UAS7) and categorized as mild, moderate, or severe. Detailed demographic and clinical data were recorded. Serum 25-hydroxyvitamin D [25(OH)D] levels were measured using a standardized chemiluminescent immunoassay and categorized as deficient (<20 ng/mL), insufficient (20–29 ng/mL), or sufficient (≥30 ng/mL). Additional routine laboratory tests were performed to exclude major confounders. Results: The majority of patients were aged 31–45 years (40.00%) and female (54.29%). Vitamin D deficiency was observed in 54.29% of patients and insufficiency in 31.43%, while only 14.29% had sufficient levels. Overall mean serum 25(OH)D was 21.07 ± 8.52 ng/mL. CSU severity distribution showed 25.71% mild, 40.00% moderate, and 34.29% severe cases, with a mean UAS7 of 22.84 ± 8.15. Mean vitamin D levels decreased progressively with increasing CSU severity: 27.54 ± 4.12 ng/mL (mild), 19.86 ± 3.98 ng/mL (moderate), and 14.72 ± 3.44 ng/mL (severe) (ANOVA p = 0.001). A significant negative correlation was found between serum vitamin D levels and UAS7 scores (r = –0.62, p = 0.001). Conclusion: Vitamin D deficiency and insufficiency are highly prevalent in CSU and are significantly associated with increased disease severity. Assessment and optimization of vitamin D status may represent a useful adjunct in the comprehensive management of CSU, although interventional studies are required to confirm causality.