This was a cross-sectional study that aimed to evaluate the association between breastfeeding and motor development of infants at 3-4 months of age in the city of Pelotas, Rio Grande do Sul. The outcome, infant motor development, was evaluated through the Bayley Scale of Infant Development (Bayley III). The prevalence of infants that stopped breastfeeding before reaching 3-4 months of age was 16.6%. The most prevalent reason for early weaning was 'insufficient milk flow' (50.4%). The general mean of motor development was 104.9 (±13.2) points, and the lowest means were attributed to babies who never breastfed (101.5; SD ±14.6), followed by those stopped breastfeeding (102.5; SD ±14.1). The results revealed that infants who never breastfed and those who had stopped at 3-4 months of age presented -1.9 (CI 95% -3.8;-0.2) points in the motor development scale means when compared with infants who were still breastfeeding.