Objective: In this study, we aimed to determine the prevalence and clinical importance of vitamin D deficiency in children and adolescent patients who were hospitalized with the diagnosis of COVID-19. Material and Methods: 40 patients who were diagnosed to have COVID-19 and hospitalized with the real-time reverse transcription polymerase chain reaction (RT-PCR) method were included. The control group consisted of 45 healthy patients with normal vitamin D levels. The age of admission, clinical and laboratory data, and 25-hydroxycholecalciferol (25-OHD) and parathormone (PTH) levels were recorded. Those with vitamin D levels which are below 20 ng/ml were determined as Group 1 and those with ≥20 ng/ml as Group 2. Results: The median levels of vitamin D level were 13.14 (4.19-69.28) in the group of patients with COVID-19 and 34.81(3.8-77.42) in the control group. Compared to the control group, there was a statistically significantly lower vitamin D level (p <0.001) in the COVID-19 patient group. At admission, the symptom of fever was significantly higher in Group 1 than in Group 2 (p =0.038). The distribution of disease severity according to vitamin D levels was not found significantly different. In conclusion; our study is the first to evaluate vitamin D levels and its relationship with clinical findings in pediatric patients diagnosed with COVID-19. There are significantly lower levels of vitamin D in children with COVID-19 than those in the control group. This shows that vitamin D, which is effective in the immunological mechanism, also has an effect in the physiopathology of the disease.