The aim of this open non-controlled clinical trial was to investigate the feasibility and preliminary treatment effects of a new group-based skills training program for adult ADHD: Group Therapy for Improving Emotional Acceptance and Regulatory Skills in Adults with ADHD (GEARS). Out of the 226 allocated patients, 213 initiated the treatment and 170 (80%) were considered treatment completers (i.e., attending at least 9 out of 14 sessions). A total of 158 of the completers also completed pre-and post-treatment assessments, and 126 provided assessments at the 3-month follow-up. Treatment credibility received high ratings, as did treatment satisfaction. Preliminary treatment effects showed significant improvements in emotion regulation with large effect sizes. Significant effects were also found for all secondary outcome variables: ADHD symptom levels, depression, anxiety, and quality of life. All significant effects remained at the 3-month follow-up. In conclusion, GEARS should be considered a feasible treatment in an outpatient psychiatric setting. The results also indicate that GEARS could be an effective treatment for adults with ADHD who are also experiencing problems with emotion regulation. However, a randomized controlled trial is needed to further evaluate the effectiveness of this new treatment.