Recent research has focused on identifying neural markers associated with risk for anxiety, including the error-related negativity (ERN). An elevated ERN amplitude has been observed in anxious individuals from middle childhood onward and has been shown to predict risk for future increases in anxiety development. The ERN is sensitive to environmental influences during development, including interpersonal stressors. Of note, one particular type of interpersonal stressor, relational victimization, has been related to increases in anxiety in adolescents. We tested whether relational victimization predicts increases in the ERN and social anxiety symptoms across two years in a sample of 152 child and adolescent females (ages 8 – 15). Results indicated that children and adolescents’ baseline ERN was positively related to the ERN two years later. Furthermore, greater relational victimization at baseline predicted greater increases in the ERN two years later, controlling for baseline ERN. Moreover, relational victimization at baseline predicted increases in social anxiety, and this relationship was mediated by increases in the ERN. These results suggest that relational victimization impacts the developmental trajectory of the neural response to errors and thereby impacts increases in social anxiety among children and adolescents.