Iron deficiency anemia (IDA) is prevalent in exclusively breastfed (EBF) Thai infants. However, in Thailand, iron supplementation guidelines for EBF infants are not available. This prospective open-label study aimed to estimate the prevalence of IDA in 9-month-old EBF infants after receiving iron supplementation from 4 months of age until they consumed adequate iron-rich food. Forty-seven healthy, 4-month-old EBF infants were prescribed 1 mg/kg/day ferrous sulfate. Their complementary food records from 6 to 9 months were calculated for daily iron intake. Complete blood count and iron study were performed at 9 months of age. The results showed that the prevalence of IDA at 9 months was 6.4%. An employed caregiver and the male sex of the baby were significantly associated with iron deficiency. The food record revealed that the median daily iron intake was less than the Thai dietary reference intake recommendation. In summary, the prevalence of IDA in infants with 9 months of exclusive breastfeeding who received iron supplementation was lower than the background rate (25.7%) when iron was not prescribed. Most infants did not have adequate iron in complementary foods. Iron supplementation should be prescribed routinely during 4-9 months of age in healthy Thai EBF infants.© 2024. The Author(s).