Importance: Several health benefits of vitamin D have been suggested; however, the safety of high-dose supplementation in early childhood is not well described. Objective: To systematically assess the risk of adverse events after high-dose supplementation with vitamin D reported in published randomized clinical trials. Data Sources: PubMed and ClinicalTrials.gov were searched through August 24, 2021. Study Selection: Randomized clinical trials of high-dose vitamin D supplementation in children aged 0 to 6 years, defined as greater than 1000 IU/d for infants (aged 0-1 year) and greater than 2000 IU/d for children aged 1 to 6 years. Data Extraction and Synthesis: Following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) reporting guideline, 2 reviewers independently extracted the data from the eligible studies. Summary risk ratio (RR), 95% CI, and P values were derived from random-effects meta-analysis. Main Outcomes and Measures: Adverse events, serious adverse events (SAEs), and/or levels of 25-hydroxyvitamin D, calcium, alkaline phosphatase, phosphate, parathyroid hormone, and/or the ratio of urine calcium to creatinine levels. Results: A total of 32 randomized clinical trials with 8400 unique participants were included. Different clinical outcomes of children receiving high-dose vitamin D supplements ranging from 1200 to 10000 IU/d and bolus doses from 30000 IU/week to a single dose of 600000 IU were evaluated. Eight studies with 4612 participants were eligible for meta-analysis using a control group receiving either low-dose vitamin D supplementation (≤400 IU/d) or placebo when investigating the risk of SAEs such as hospitalization or death. No overall increased risk of SAEs in the high-dose vitamin D vs control groups was found (RR, 1.01 [95% CI, 0.73-1.39]; P =.89, I²= 0%). In addition, risk of hypercalcemia (n = 726) was not increased (RR, 1.18 [95% CI, 0.72-1.93]; P =.51). Clinical adverse events potentially related to the vitamin D supplementation reported in the studies were rare. Conclusions and Relevance: This meta-analysis and systematic review found that high-dose vitamin D supplementation was not associated with an increased risk of SAEs in children aged 0 to 6 years, and that clinical adverse events potentially related to the supplementation were rare. These findings suggest that vitamin D supplementation in the dose ranges of 1200 to 10000 IU/d and bolus doses to 600000 IU to young children may be well tolerated..