Piglets are born with very low levels of vitamin D. Feed is the only source of vitamin D for pigs kept indoors, and the levels in feed are restricted by European legislation. We aimed to study the effect of lamps releasing ultraviolet type B (UVB) light on the vitamin D status (serum 25-hydroxyvitamin D) in sows and piglets in a Danish indoor herd. A randomized trial with a parallel group design was initiated with two groups receiving a daily UVB-dose of maximum 0.7 standard erythema dose (SED) or 1 SED, in addition to a control group. The three groups included in the study consisted of 15 sows and their 195 offspring. Blood samples were taken from the piglets and sows on day 1, 12, and 24. Results showed no difference between the groups in serum levels of 25(OH)D3 or vitamin D3 on day 1, with the mean (±SD) for piglets being 0.96 ± 0.26 ng/mL and 0.06 ± 0.04 ng/mL, respectively. For sows, the values were 16 ± 3 ng/mL 25(OH)D3 and 3 ± 0.8 ng/mL vitamin D3 on day 1. A significant difference (p < 0.001) in serum 25(OH)D3 between the groups receiving UVB light and the control group was observed on both day 12 and day 24. On day 24, the piglet control group had 5.5 ± 2 ng/mL 25(OH)D3 and 0.4 ± 0.2 ng/mL vitamin D3. For the UVB groups, the values were 21.6 ± 10. ng/mL 25(OH)D3 and 8.3 ± 2.5 ng/mL vitamin D3 for the 0.7 SED group and 19.5 ± 6.0 ng/mL 25(OH)D3 and 7.6 ± 3.4 ng/mL vitamin D3 for the 1 SED group. For the sows, the values were 25.6 ± 5.5 ng/mL 25(OH)D3 and 6.6 ± 1.2 ng/mL vitamin D3 for the control group, 66.7 ± 13.5 ng/mL 25(OH)D3 and 21.3 ± 2.9 ng/mL vitamin D3 for 0.7 SED group and 67 ± 15 ng/mL 25(OH)D3 and 25 ± 5 ng/mL vitamin D3 for the 1 SED. No significant difference was found between the two UVB groups for either piglets or sows. The use of lamps releasing UVB light is therefore suggested to be an efficient way to improve the vitamin D status of both sows and piglets.