Severe iodine deficiency (ID) during gestation is associated with neurocognitive sequelae. The long-term impact of mild ID, however, has not been well characterized. The purpose of this study was to determine whether children born to mothers with urinary iodine concentrations (UICs) <150 μg/L during pregnancy have poorer educational outcomes in primary school than peers whose mothers did not have gestational ID (UIC ≥150 μg/L). This was a longitudinal follow-up (at 9 years old) of the Gestational Iodine Cohort. Pregnancy occurred during a period of mild ID in the population, with the children subsequently growing up in an iodine-replete environment. Participants were children whose mothers attended The Royal Hobart Hospital (Tasmania) antenatal clinics between 1999 and 2001. Australian national curriculum and Tasmanian state curriculum educational assessment data for children in year 3 were analyzed. Children whose mothers had UIC <150 μg/L had reductions of 10.0% in spelling (-41.1 points, 95% confidence interval [CI], -68.0 to -14.3, P = .003), 7.6% in grammar (-30.9 points, 95% CI, -60.2 to -1.7, P = .038), and 5.7% in English-literacy (-0.33 points, 95% CI, -0.63 to -0.03, P = .034) performance compared with children whose mothers' UICs were ≥150 μg/L. These associations remained significant after adjustment for a range of biological factors (maternal age at birth of child, gestational length at time of birth, gestational age at time of urinary iodine collection, birth weight, and sex). Differences in spelling remained significant after further adjustment for socioeconomic factors (maternal occupation and education). This study provides preliminary evidence that even mild iodine deficiency during pregnancy can have long-term adverse impacts on fetal neurocognition that are not ameliorated by iodine sufficiency during childhood.