Background: Dysfunctional emotion regulation (ER) is associated with symptoms of depression and anxiety in adolescents. This pilot study aimed to examine the acceptability and feasibility of a guided internet-based emotion regulation training (ERT) added to cognitive behavioral therapy (CBT). Furthermore, we aimed to examine the feasibility of the randomized study design and to provide a first estimate of the effectiveness of CBT + ERT compared with CBT alone in adolescents with depressive or anxiety disorders.Methods: In a pilot randomized controlled trial (RCT) with a parallel group design, 39 patients (13-18 years) with depressive or anxiety disorder were assigned to CBT + ERT (n = 21) or CBT (n = 18). Assessments at baseline, three-months and six-months follow-up included treatment adherence, satisfaction, depressive symptoms, anx-iety symptoms, and ER strategies.Results: Adherence to ERT was 66.5 %, and treatment satisfaction was adequate. 76.5 % of eligible patients participated in the study. Linear mixed-model analyses showed significantly reduced anxiety symptoms (p = .003), depressive symptoms (p = .017), and maladaptive ER (p = .014), and enhanced adaptive ER (p = .008) at six months follow-up in the CBT + ERT group compared to controls. Limitations: The sample size was small, and results regarding effectiveness remain preliminary. Data-collection took place during COVID-19, which may have influenced the results.Conclusions: Both the intervention and the study design were found to be feasible. In a larger RCT, however, improvement of recruitment strategy is necessary. Preliminary results indicate potential effectiveness in decreasing anxiety, depression, and emotion dysregulation in adolescents. The next step should be the devel-opment of an improved internet-based ERT and its evaluation in a larger RCT.Trial registration: Registered on January 14th, 2020 in The Netherlands Trial Register (NL8304).