CONTEXT:: Iodine deficiency is the leading cause of preventable neurodevelopmental delay in children worldwide and a possible public health concern in Haiti. OBJECTIVE:: To determine the prevalence of iodine deficiency in Haitian young children and its influence by environmental factors. DESIGN:: Cross-sectional study, March through June 2015. SETTING:: Community churches in 3 geographical regions in Haiti. PARTICIPANTS:: 299 healthy Haitian children aged 9 months to 6 years; one-third each enrolled in a coastal, mountainous, and urban region. MAIN OUTCOME MEASURES:: Urinary iodide, serum thyrotropin (TSH), goiter assessment, and urinary perchlorate and thiocyanate. RESULTS:: Mean age was 3.3±1.6 years, with 51% female, median family income USD 30/week, and 16% malnutrition. Median urinary iodide levels were normal in coastal (145 μg/L, interquartile range [IQR] 97 to 241) and urban regions (187 μg/L, IQR 92 to 316), but revealed mild iodine deficiency in a mountainous region (89 μg/L, IQR 56 to 129), P < 0.0001. Grade 1 goiters were palpated in 2 children, but TSH values were normal. Urinary thiocyanate and perchlorate concentrations were not elevated. Predictors of higher urinary iodide included higher urinary thiocyanate and perchlorate, breastfeeding, and not living in a mountainous region. CONCLUSIONS:: Areas of mild iodine deficiency persist in Haiti’s mountainous regions. Exposure to two well-understood environmental thyroid function disruptors is limited.