Background Despite extensive literature on the role of breastfeeding in maternal and child health and cognitive development, few studies have systematically tested whether breastfeeding predicts children's socio‐emotional outcomes. The present study examined associations between trajectories of breastfeeding and observed parent–child interaction qualities of maternal sensitivity, child positivity, and child negativity from 6 months to 3 years of age. Methods Data were drawn from the NICHD Study of Early Child Care and Youth Development (n = 1306 US families). Hierarchical linear modelling accounted for demographic and early characteristics, including home environment, maternal depression, and observed global relationship quality. Results Breastfeeding was associated with increases in observed maternal sensitivity over time, even after the effects of demographic and early characteristics were controlled. Accounting for the covariates, breastfeeding was not associated with child behaviour (i.e. positivity, negativity) in mother–child interaction across early childhood. Conclusions Improved relationship quality, specifically through changes in maternal behaviour, may be another advantage experienced by breastfeeding mothers and children.