Abstract Background Low levels of circulating 25-hydroxy-vitamin D [25(OH)D] have been shown to associate with prevalent attention-deficit/hyperactivity disorder (ADHD), but few studies have examined the association between 25(OH)D during fetal development and risk of childhood ADHD. Methods Maternal plasma 25(OH)D was measured at 10–18 and 32–38 weeks of gestation, with sufficiency defined as 25(OH)D ≥ 30 ng/ml. Offspring ADHD status between ages 6–9 years was measured by parent report of clinical ADHD diagnosis among 680 mother-child pairs from the Vitamin D Antenatal Asthma Reduction Trial. Association between maternal 25(OH)D and child ADHD was assessed using logistic regression, adjusting for maternal age, race and ethnicity. Effect modification by offspring sex was also assessed. Results No associations between maternal 25(OH)D at 10–18 weeks of gestation and offspring ADHD were observed. In the third trimester, we observed associations between maternal vitamin D sufficiency and offspring ADHD [odds ratio (OR) 0.47, 95% confidence interval (CI) 0.26–0.84], in addition to maternal 25(OH)D sufficiency category, comparing the deficient (OR 0.34, 95% CI 0.12–0.94), insufficient (OR 0.41, 95% CI 0.15–1.10) and sufficient (OR 0.20, 95% CI 0.08–0.54) categories against highly deficient 25(OH)D, respectively. Stratified analyses revealed a protective association for sufficient maternal 25(OH)D and child ADHD among males (OR 0.47, 95% CI 0.23–0.94); the synergy index for additive effect modification of risk was 1.78 (95% CI 0.62–5.08). Conclusions Higher levels of maternal vitamin D in the third trimester are associated with lower risk of ADHD in offspring, with modest evidence for a stronger effect among male offspring. However, larger studies will be necessary to confirm these findings.