Human breast milk dynamically adapts to meet the needs of healthy neurodevelopment. While a great deal of research has examined the relationship between breastfeeding, infant cognitive development and IQ, findings are inconclusive when potential confounders are adjusted for. This raises questions about the various ways in which breastfeeding and other crucial factors can impact on infant IQ and cognitive abilities. The main aim of this study was to analyse the relationship between breastfeeding and child IQ and cognitive abilities after adjusting for sociodemographic, perinatal and postnatal variables. The participants were 613 boys and girls aged 4-5 years old from two cohort studies carried out in Tarragona, Spain. IQ and cognitive abilities were assessed using the Wechsler Preschool and Primary Scale of Intelligence-IV (WPPSI-IV). Sociodemographic, prenatal, perinatal, and postnatal factors were collected. Descriptive analyses compared mother and infant characteristics by breastfeeding categories using Chi-squared and ANOVA tests. Regression models explored associations between breastfeeding duration and WPPSI-IV indexes. Breastfeeding from 1 to 8 months was related to higher scores on the Full-Scale IQ (b = 3.909, p = 0.035) and Working Memory Index (b = 3.757, p = 0.044), Non-Verbal Index (b = 4.184, p = 0.029), Cognitive Proficiency Index (b = 4.015, p = 0.038) at 4-5 years old, even after adjusting for the mother's IQ and mother-infant attachment difficulties. Our study emphasizes the numerous advantages of breastfeeding and underscores the importance of infants being breastfed through healthcare and health policies. Advocating breastfeeding for the first six months of life is crucial for enhancing lifelong well-being and cognitive development in children.