Maternal psychosocial stress during pregnancy can adversely influence child development, but few studies have investigated psychosocial stress during the postpartum period and its association with risk of toddler developmental delays. Moreover, given the expanding diversity of the U.S. population, and well-documented health and stress disparities for racial and ethnic minorities, research examining the effect of postpartum stress on risk of developmental delays in diverse populations is of critical importance. In this study, data from the Community Child Health Network provided the opportunity to test maternal postpartum stress as a predictor of toddler risk of developmental delay in a sample of African American, Latina and non-Hispanic White women and their toddlers (N = 1537) recruited in urban, suburban, and rural communities. Postpartum maternal stress over 1 year was operationalized as perceived stress, life events, and negative impact of life events. Regression results revealed higher risk of developmental delays in toddlers whose mothers experienced more negative life events, greater negative impact of events, and higher perceived stress over the year. Prenatal stress, pregnancy/birth complications, and postpartum depression did not explain these associations. Maternal postpartum stress may contribute to increased risk for developmental delays and is an important target for psychosocial intervention.