Background: Major Depressive Disorder is one of the most challenging mental health problems of our time. Although effective psychotherapeutic treatments are available, many patients fail to demonstrate clinically significant improvements. Difficulties in emotion regulation have been identified as putative risk and maintaining factors for Major Depressive Disorder. Systematically enhancing adaptive emotion regulation skills should thus help reduce depressive symptom severity. However, at this point, no study has systematically evaluated effects of increasing adaptive emotion regulation skills application on symptoms of Major Depressive Disorder. In the intended study, we aim to evaluate stand-alone effects of a group-based training explicitly and exclusively targeting general emotion regulation skills on depressive symptom severity and assess whether this training augments the outcome of subsequent individual cognitive behavioral therapy for depression.Methods/Design: In the evaluation of the Affect Regulation Training, we will conduct a prospective randomized-controlled trial. Effects of the Affect Regulation Training on depressive symptom severity and outcomes of subsequent individual therapy for depression will be compared with an active, common factor based treatment and a waitlist control condition. The study sample will include 120 outpatients meeting criteria for Major Depressive Disorder. Depressive symptom severity as assessed by the Hamilton Rating Scale will serve as our primary study outcome. Secondary outcomes will include further indicators of mental health and changes in adaptive emotion regulation skills application. All outcomes will be assessed at intake and at 10 points in time over the course of the 15-month study period. Measures will include self-reports, observer ratings, momentary ecological assessments, and will be complemented in subsamples by experimental investigations and the analysis of hair steroids.Discussion: If findings should support the hypothesis that enhancing regulation skills reduces symptom severity in Major Depressive Disorder, systematic emotion regulation skills training can enhance the efficacy and efficiency of current treatments for this severe and highly prevalent disorder.Trial registration: This study is registered with ClinicalTrials.gov, number NCT01330485.