Increased neural error monitoring, as measured by the error-related negativity (ERN), is a transdiagnostic neurobiological marker of anxiety. To date, little is known about whether the ERN can inform the choice between first-line anxiety disorder treatments and whether the ERN changes following treatment completion. The aim of the study was to therefore assess whether the ERN is a treatment moderator and index of symptom change during cognitive-behavioral therapy (CBT) or selective serotonin reuptake inhibitors (SSRIs). Participants included adult volunteers (M age=25.8±8.5; 67% female) with principal anxiety disorders (n=60) or no lifetime history of Axis I psychopathology (ie, healthy controls; n=26). A flanker task was used to elicit the ERN at baseline and 12 weeks later, following either CBT or SSRIs in the patient sample. Results indicated that baseline ERN was a significant treatment moderator such that a more enhanced baseline ERN was associated with greater reduction in anxiety symptoms within individuals who received CBT but not SSRIs. Results also revealed that the ERN increased pre- to post-treatment among patients randomized to SSRIs, but remained stable among patients randomized to CBT and healthy controls. Together, these novel findings highlight that ERN may help guide treatment decisions regarding engagement in CBT or SSRIs, especially among individuals with an enhanced ERN. The findings also suggest that SSRIs have the capacity to alter individual differences in the ERN, providing evidence that the ERN is not entirely static in patients with anxiety disorders.