A multifaceted, relationally focused intervention involving group and individual pre‐ and postnatal counseling, print resources, and community resources encouraged 390 fathers of newborn infants in Vietnam to responsively support mothers and work with them as a parenting team. Both partners completed questionnaires prebirth and 1‐, 4‐, and 9‐months postbirth on measures of breastfeeding support, exclusive breastfeeding duration, relationship quality, and infant development. Compared to 412 comparison group couples, intervention couples evidenced greater father support, especially in terms of helping and responsiveness to the mother's needs. This support predicted longer exclusive breastfeeding duration, improved relationship quality, and higher levels of infant development at 9 months. Sensitively working together with mothers as a coordinated team enhanced couple's relationship functioning and improved children's developmental outcomes.