Background: Nephrotic syndrome (NS) is a kidney disorder characterized by excessive protein loss in the urine, hypoalbuminemia, hyperlipidemia, and generalized edema. It primarily affects children but can also occur in adults.This study aims to evaluate changes in bone mineral density (BMD) and the role of vitamin D and calcium supplementation in patients undergoing corticosteroid therapy for the first episode of nephrotic syndrome. Material and Methods: A total of 80 patients aged 2 to 18 years with the first episode of nephrotic syndrome were included in this study. Patients were randomized into two groups: one group received vitamin D (800–1,000 IU/day) and calcium (500–1,000 mg/day) supplementation, while the control group received only dietary counseling. All patients underwent standard corticosteroid therapy with prednisolone for 12 weeks. Bone mineral density (BMD) was measured using dual-energy X-ray absorptiometry (DEXA) at baseline, 3 months, and 6 months. Serum calcium, phosphorus, alkaline phosphatase, parathyroid hormone (PTH), and 25-hydroxyvitamin D levels were also assessed. Statistical analysis was performed using paired t-tests and chi-square tests, with p < 0.05 considered statistically significant. Results: At baseline, BMD was identical in both groups (lumbar spine: 0.78 ± 0.05 g/cm², femoral neck: 0.69 ± 0.04 g/cm²). By 6 months, the supplemented group demonstrated a significant increase in BMD (lumbar spine: 0.85 ± 0.07 g/cm², femoral neck: 0.76 ± 0.05 g/cm²), while the control group showed a decline in BMD (lumbar spine: 0.75 ± 0.07 g/cm², femoral neck: 0.66 ± 0.05 g/cm²). Serum 25(OH) vitamin D levels increased from 22.45 ± 5.62 ng/mL to 35.62 ± 7.10 ng/mL in the supplemented group, whereas PTH levels decreased from 48.67 ± 8.34 pg/mL to 42.89 ± 7.45 pg/mL. Fracture incidence was lower in the supplemented group (2.50%) compared to the control group (5.00%), and BMD reduction was significantly lower in the supplemented group (3.20% vs. 7.80%, p = 0.012). Conclusion: Vitamin D and calcium supplementation significantly improved BMD and biochemical markers of bone metabolism in corticosteroid-treated nephrotic syndrome patients. The supplemented group showed better bone preservation, reduced BMD loss, and lower fracture incidence compared to the control group. These findings suggest that routine supplementation should be considered to prevent corticosteroid-induced bone deterioration in nephrotic syndrome patients.