OBJECTIVE: To investigate whether circulating 25-hydroxyvitamin D3 [25(OH)D3] concentration in pregnancy is associated with neuropsychological development in infants. METHODS: The Spanish population-based cohort study INfancia y Medio Ambiente Project recruited pregnant women during the first trimester of pregnancy between November 2003 and February 2008. Completed data on 1820 mother-infant pairs were used. Maternal plasma 25(OH)D3 concentration was measured by high-performance liquid chromatography in pregnancy (mean 13.5±2.1 weeks of gestation). Offspring mental and psychomotor scores were assessed by trained psychologists at age 14 months (range, 11–23) by using the Bayley Scales of Infant Development. β-Coefficients with 95% confidence intervals (CIs) of mental and psychomotor scores associated with continuous or categorical concentrations of maternal plasma 25(OH)D3 were calculated by using linear regression analysis. RESULTS: The median plasma value of 25(OH)D3 in pregnancy was 29.6 ng/mL (interquartile range, 21.8–37.3). A positive linear relationship was found between circulating concentrations of maternal 25(OH)D3 concentrations in pregnancy and mental and psychomotor scores in the offspring. After adjustment for potential confounders, infants of mothers with 25(OH)D3 concentrations in pregnancy >30 ng/mL showed higher mental score (β = 2.60; 95% CI 0.63–4.56) and higher psychomotor score (β = 2.32; 95% CI 0.36–4.28) in comparison with those of mothers with 25(OH)D3 concentrations <20 ng/mL. CONCLUSIONS: Higher circulating concentration of maternal 25(OH)D3 in pregnancy was associated with improved mental and psychomotor development in infants.