STUDY OBJECTIVE: To explore the association between maternal social capital and child physical and mental health in Vietnam. DESIGN: Cross sectional survey. Measures of maternal structural social capital comprised group membership, citizenship, and social support. Measures of cognitive social capital comprised trust, social harmony, sense of fairness, and belonging. Child health was measured by anthropometrics and mothers' reports of acute and chronic physical health problems and child mental health. PARTICIPANTS: 2907 mothers and their 1 year old or 8 year old children from five provinces in Vietnam. MAIN RESULTS: The study found low levels of group membership and citizenship and high levels of cognitive social capital and support, and generally higher levels of social capital among the mothers of 8 year old compared with 1 year old children. All but one association was in the hypothesised direction (that is, higher levels of social capital associated with reduced risk of child health problems). There were more statistically significant relations between maternal social capital and the health of 1 year olds compared with 8 year old children, and between measures of social support and cognitive social capital and child health, than with group membership and involvement in citizenship activities. CONCLUSION: This study is the first to explore the association between multiple dimensions of social capital and a range of different child health outcomes in the developing world. These results now need to be tested using longitudinal data.