Background Risk of preeclampsia varies by month of delivery. We tested whether this seasonal patterning may be mediated through maternal vitamin D concentration using antenatal exposure to UV-B radiation as an instrumental variable. Methods and Results Scottish maternity records were linked to antenatal UV-B exposure derived from satellites between 2000 and 2010. Logistic regression analyses were used to explore the association between UV-B and preeclampsia, adjusting for the potential confounding effects of month of conception, child's sex, gestation, parity, and mean monthly temperature. Of the 522 896 eligible singleton deliveries, 8689 (1.66%) mothers developed preeclampsia. Total antenatal UV-B exposure ranged from 43.18 to 101.11 kJ/m2 and was associated with reduced risk of preeclampsia with evidence of a dose-response relationship (highest quintile of exposure: adjusted odds ratio, 0.57; 95% CI, 0.44-0.72; P<0.001). Associations were demonstrated for UV-B exposure in all 3 trimesters. Conclusions The seasonal patterning of preeclampsia may be mediated through low maternal vitamin D concentration in winter resulting from low UV-B radiation. Interventional studies are required to determine whether vitamin D supplements or UV-B-emitting light boxes can reduce the seasonal patterning of preeclampsia.