Recent literature has highlighted the possible role of vitamin D in atopic dermatitis (AD), and that vitamin D supplementation might help to treat AD. This study determined the relationship between vitamin D level and AD, and assessed the efficacy of vitamin D supplementation. We searched the MEDLINE, EMBASE, and Cochrane databases up to May 2015. Observational studies and randomized controlled trials were included based on the available data on the serum 25-hydroxyvitamin D (25(OH)D) level and quantified data available for severity assessed using the Scoring Atopic Dermatitis (SCORAD) index or Eczema Area and Severity Index (EASI) score. Compared with healthy controls, the serum 25(OH)D level was lower in the AD patients of all ages (standardized mean difference = -2.03 ng/mL; 95% confidence interval (CI) = -2.52 to -0.78), and predominantly in the pediatric AD patients (standardized mean difference = -3.03 ng/mL; 95% CI = -4.76 to -1.29). In addition, the SCORAD index and EASI score decreased after vitamin D supplementation (standardized mean difference = -5.85; 95% CI = -7.66 to -4.05). This meta-analysis showed that serum vitamin D level was lower in the AD patients and vitamin D supplementation could be a new therapeutic option for AD.