--- - |2+ Objective An increased neural response to making mistakes has emerged as a potential biomarker of anxiety across development. The error‐related negativity (ERN) is an event‐related potential elicited when people make mistakes on simple laboratory‐based reaction time tasks that has been associated with risk for anxiety. This study examined whether the ERN prospectively predicted the first onset of generalized anxiety disorder (GAD) over 1.5 years in adolescent girls. Methods The sample included 457 girls between the ages of 13.5 and 15.5 years, with no history of GAD. At baseline, the ERN was measured using a flankers task. Psychiatric history of the adolescent and biological parent was assessed with diagnostic interviews, and the adolescent completed a self‐report questionnaire regarding anxiety symptoms. Approximately 1.5 years later, adolescents completed the same interview. Results An increased neural response to errors at baseline predicted first‐onset GAD over 1.5 years. The ERN was a significant predictor independent of other prominent risk factors, including baseline anxiety and depression symptoms and parental lifetime psychiatric history. Jointly the ERN and social anxiety symptoms provided the greatest power for predicting first‐onset GAD. Conclusions This study provides evidence for the utility of the ERN as a biomarker of risk for GAD during a key developmental period. - Journal of Child Psychology and Psychiatry, Volume 59, Issue 11, Page 1162-1170, November 2018.