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Although iodine deficiency during pregnancy has been linked to poorer childhood neurodevelopment, it is unclear whether these associations persist into adolescence. We examined whether first trimester iodine is associated with children's intelligence at age 15. We used data from 1,211 mother–child pairs in the Avon Longitudinal Study of Parents and Children (ALSPAC), with first-trimester urinary iodine-to-creatinine ratios during pregnancy and intelligence quotient (IQ) scores of the child at age 15, as assessed from the Two-Subtest Wechsler Abbreviated Scale of Intelligence. Associations were tested using linear regression adjusted for confounders. Iodine status was analysed both continuously and categorically using cut-offs (< 50, 50–99, 100–149, 150–249, > 250 µg/g). We tested interactions with child sex and maternal thyroid function. Lower iodine was associated with lower Vocabulary T-scores (β = 0.72; 95% CI: 0.09–1.35), but not Matrix Reasoning or Full-Scale IQ. Children of mothers with severe iodine deficiency (< 50 µg/g) had 4.25 points lower Vocabulary T-scores (Cohen’s d = –0.40) and 3.3 points lower Full-Scale IQ (Cohen’s d = –0.27) than those with sufficient iodine status (150–250 µg/g). The associations did not differ by sex. Iodine status in the first trimester was not correlated with maternal thyroid function. However, in iodine-deficient pregnancies (< 150 µg/g), higher maternal thyroid-stimulating hormone (TSH) was associated with higher Vocabulary and Full-Scale IQ scores, a pattern not seen in iodine-sufficient pregnancies. Iodine deficiency in early pregnancy is associated with lower adolescent verbal intelligence and, in severe iodine deficiency, also with reduced full-scale intelligence. The observed association between higher maternal TSH and higher verbal and full-scale intelligence in iodine-deficient pregnancies suggests a possible compensatory role of the maternal thyroid axis. Adequate periconceptional iodine intake remains important and merits further study.