For many reptile species, adequate ultraviolet B (UVB) radiation is essential for proper calcium metabolism. In this study, the effects of UVB radiation on calcium metabolism were evaluated in green sea turtles (Chelonia mydas). 25-Hydroxyvitamin D, parathyroid hormone (PTH), and ionized calcium (iCa) were measured in juvenile (n = 18, 9 indoor, 9 outdoor) and adult (n = 8, 4 indoor, 4 outdoor) turtles. All animals were fed an identical diet. Outdoor animals had access to unfiltered sunlight, whereas indoor animals were housed under artificial lighting without UVB. Mean values for 25-hydroxyvitamin D for the outdoor and indoor groups were 34.33 ± 7.98 nmol/L and 7.11 ± 1.69 nmol/L for juveniles and 73.25 ± 30.34 nmol/L and 14.0 ± 11.52 nmol/L for adults respectively. Mean values for iCa for the outdoor and indoor groups were 0.98 ± 0.07 mmol/L and 0.99 ± 0.06 mmol/L for juveniles and 1.18 ± 0.22 mmol/L and 0.97 ± 0.18 mmol/L for adults respectively. UVB exposure (P < 0.001) and age (P < 0.001) had a significant effect on 25-hydroxyvitamin D as well as a significant interaction between the two variables (P = 0.008), with highest values in adult outdoor turtles. There was a significant interaction between age group and UVB status for iCa (P = 0.036), with greater values in older outdoor turtles. 25-Hydroxyvitamin D and total calcium were positively correlated, rs = 0.39, P = 0.042. iCa and calcium-to-phosphorus ratios were also positively correlated, rs = 0.42, P = 0.027. These results suggest that UVB exposure is an important source of 25-hydroxyvitamin D for green sea turtles and has significant effects on calcium metabolism in this species. PTH values in this study were near the minimum limits of detection and suggest that current mammalian-based PTH assays are not valid for reptiles.