Background: Vitamin D deficiency is increasingly recognized as a global health concern, with potential implications for cancer development and progression. This systematic review investigated the prevalence of vitamin D deficiency in pediatric cancer patients and its potential impact on clinical outcomes. Methods: A comprehensive literature search was conducted across multiple databases, including PubMed, Web of Science, and Cochrane Library, to identify the relevant studies published between 2009 and July 2024. Studies were included if they assessed vitamin D status in pediatric cancer patients and reported on the clinical outcomes. Data extraction and quality assessment were performed independently by two reviewers. Results: The review included 20 original articles encompassing a diverse pediatric population with various cancer types. A high prevalence of vitamin D deficiency was observed across the studies. Deficiency was associated with older age and lower socioeconomic status. Several studies reported associations between vitamin D deficiency and the increased risk of infection, poorer treatment response, and decreased survival. Conclusions: Vitamin D deficiency is highly prevalent in pediatric cancer patients and may negatively impact clinical outcomes. Routine screening for vitamin D deficiency and personalized supplementation strategies should be considered in this population. Further research is needed to establish optimal vitamin D management protocols and evaluate the long-term benefits of vitamin D repletion in pediatric oncology.