Exposure to destructive interparental conflict consistently predicts children's externalizing symptoms. Research has identified children's emotional security as an explanatory mechanism underpinning this association, but little is known about the role of children's neurophysiology in this pathway. We aimed to address that gap using event-related potential (ERP) data from a sample of 86 children, ages 9-11 years. The P3 ERP was measured during an emotion categorization task involving photographs of a couple exhibiting facial expressions of emotion toward one another. Angry images were target stimuli, and happy and neutral images were nontarget stimuli. Children reported interparental conflict, and their mothers reported children's emotional security. Mothers also reported children's externalizing symptoms at two time points 1 year apart. Results indicated elevated interparental conflict predicted more behavioral dysregulation (an aspect of emotional insecurity), which predicted a smaller P3 on angry trials, which, in turn, predicted greater externalizing symptoms 1 year later, controlling for initial externalizing. Greater involvement in interparental conflict, another aspect of emotional insecurity, predicted a larger P3 on neutral trials. Further, both greater involvement and a larger P3 on happy trials predicted increases in externalizing. The results suggest differing associations between different elements of emotional insecurity and the P3.