Deficits in general emotion regulation skills have been shown to be associated with various mental disorders. Thus, general affect-regulation training has been proposed as promising transdiagnostic approach to the treatment of psychopathology. In the present study, we aimed to evaluate the efficacy of a general affect-regulation as a stand-alone, group-based treatment for depression. For this purpose, we randomly assigned 218 individuals who met criteria for major depressive disorder (MDD) to the Affect Regulation Training (ART), to a waitlist control condition (WLC), or to a condition controlling for common factors (CFC). The primary outcome was the course of depressive symptom severity as assessed with the Hamilton Rating Scale for Depression and the Beck Depression Inventory. Differences between groups will be analyzed with the help of multi-level analyses. To clarify mechanisms of change we will test whether changes in emotion regulation skills will mediate potential differences between conditions regarding change of depressive symptoms. As secondary goal, the study will try and clarify whether participating in ART might augment the efficacy of subsequent individual cognitive-behavioral therapy.