Background: Dengue fever, a major public health challenge in tropical regions, shows increasing incidence and severity worldwide, with frequent outbreaks in Bangladesh. Vitamin D, known for its immune-modulating properties, has been suggested to influence outcomes in viral infections, including dengue. This study aimed to evaluate the vitamin D status among dengue patients and examine its association with disease severity and recovery outcomes. Methods: This hospital-based cross-sectional study was conducted from January 2022 to December 2022 in the Dengue cell of Bangabandhu Shekh Mujib Medical University, Dhaka, Bangladesh. Vitamin D was measured in the Department of Biochemistry and Molecular Biology, Bangabandhu Sheikh Mujib Medical University, Dhaka, Bangladesh. A total of 130 dengue patients, aged 18 to 60, were enrolled. Serum vitamin D levels were measured and analyzed alongside demographic, clinical, and biochemical parameters to assess the correlation between vitamin D status and dengue severity. Results: The mean age of patients was 35.6 years, with a predominance of males (57.7%). Vitamin D deficiency (levels <20 ng/mL) was observed in 69.2% of patients, with mean levels significantly decreasing with increased dengue severity (18.5 ng/mL in mild cases vs. 9.8 ng/mL in severe cases, p < 0.01). Patients with lower vitamin D levels had prolonged hospital stays and recovery times. A significant decrease in vitamin D levels from pre- to post-diagnosis (20.0 ng/mL to 15.2 ng/mL) was also noted, underscoring the potential impact of dengue infection on vitamin D status. Conclusions: This study highlights a high prevalence of vitamin D deficiency among dengue patients, with lower levels associated with increased disease severity and slower recovery. Targeted interventions, including vitamin D supplementation, may improve patient outcomes and warrant further investigation.