Although diverse immunomodulatory effects of vitamin D are increasingly being recognized, the ability of oral vitamin D to rapidly modulate immune responses in vivo has not been established in humans. We designed an interventional study to test the hypothesis that oral vitamin D would be capable of rapidly attenuating experimental sunburn. Twenty healthy adults were randomized, in a double-blinded fashion, to receive either placebo or a high dose of vitamin D 3 (cholecalciferol) one hour after experimental sunburn induced by an erythemogenic dose of simulated solar radiation. As compared to placebo, participants receiving vitamin D 3 (200,000 IU) demonstrated reduced expression of the pro-inflammatory mediators TNF-α (p=0.04) and iNOS (p=0.02) in skin biopsy specimens 48 hours after experimental sunburn. A blinded unsupervised hierarchical clustering of participants based on global gene expression profiles revealed that participants with significantly higher serum vitamin D 3 levels after treatment (p=0.007) demonstrated increased skin expression of the anti-inflammatory mediator arginase-1 (p=0.005), a sustained reduction in skin redness (p=0.02), and significant expression of skin barrier repair genes. In contrast, participants with lower serum vitamin D 3 levels had significant expression of pro-inflammatory genes. These findings demonstrate that oral vitamin D is capable of rapidly attenuating experimental sunburn, and implicate arginase-1 up-regulation as a novel mechanism by which vitamin D exerts anti-inflammatory effects in humans. These results have broad implications for the role of vitamin D in skin homeostasis, and suggest that oral vitamin D may be clinically therapeutic for its immunomodulatory properties.