Background/Aim: The effect of vitamin D on skin carcinogenesis is unclear. Vitamin D derivatives may protect against ultraviolet radiation (UVR)-induced DNA damage, immune suppression, and skin carcinogenesis. However, some epidemiological studies have reported an increased incidence of skin cancer associated with high serum vitamin D levels. We investigated the effect of vitamin D supplementation on serum, skin, and tumor vitamin D levels and on skin cancer development in hairless immunocompetent mice. Materials and Methods: Female C3.Cg-Hr^hr/TifBomTac immunocompetent mice (n=125) were randomly separated into five groups. Two groups received a high vitamin D₃ diet (4.5 μg/day/mouse). One group received a medium vitamin D₃ diet (2.3 μg/day/mouse). Two groups received a standard diet (0.045 µg/day/mouse). Three standard erythema doses of UVR were given three times per week to three groups. Results: Animals on a high vitamin D₃ diet had ~150-fold higher serum vitamin D₃ levels (p=0.00016) and 3-fold higher serum 25-hydroxyvitamin D₃ [25(OH)D₃] levels (p=0.00016) than those on a standard diet. For mice on the medium vitamin D₃ diet, serum vitamin D₃ and 25(OH)D₃ levels were 18-fold and 2.3-fold higher than for the standard diet, respectively (p=0.00016). All UVR-exposed mice developed tumors. Vitamin D₃ levels were lower in the tumor than the skin (p<0.0001). High and medium supplementation with vitamin D₃ did not affect tumor development (p>0.05). Conclusion: In mice, vitamin D levels in the serum, skin, and tumors were augmented by supplementation, but this did not affect the development of UVR-induced skin tumors.