This study examined the moderating role of effortful control in the association between interparental conflict and externalizing problems in a diverse sample of preschool children (N = 243; M age = 4.60 years). Using a multimethod, multi-informant, prospective design, findings indicated that the relation between interparental conflict and externalizing problems was only significant among children with poor effortful control. Children with high effortful control appeared to be protected against the negative effects of interparental conflict exhibiting low levels of externalizing problems despite increasing levels of interparental conflict. Toward identifying the mechanisms underlying the protective effects of effortful control, mediated moderation analyses indicated that children's effortful control protects children against interparental conflict by reducing their angry reactivity to interparental conflict.