BackgroundGroup psychotherapy is an effective treatment for patients with mental health issues. This study aims to evaluate data on the effectiveness of a cost-free short-term outpatient group psychotherapy project for patients with mixed mental health issues in Tyrol, Austria.MethodsIn this naturalistic observational study, outpatients taking part in the psychotherapeutic group treatment between spring 2018 and spring 2020 were included. Patients completed the patient health questionnaire (PHQ-D), an item of the working ability index (WAI) and single items on symptom burden, treatment expectation and perceived benefit before the first (T0) and/or last group session (T1). Mean changes were investigated using repeated measure analyses of variance (rANOVA).ResultsA total of 98 patients were included in the study. Statistically significant improvements with medium to large effect sizes were observed for depression (η2= 0.22,p< 0.001), somatization (η2= 0.10,p= 0.008), anxiety (η2= 0.18,p< 0.001), and subjective working ability (η2= 0.22,p< 0.001). Neither age (p= 0.85), sex (p= 0.34), baseline symptoms (p= 0.29–0.77), nor previous experience with individual (p= 0.15) or group psychotherapy (p= 0.29) were associated with treatment outcome. However, treatment expectation at baseline was significantly associated with the patients’ perception of the treatment benefit (r = 0.39,p< 0.001).ConclusionOur study highlights the benefit of outpatient short-term group psychotherapy for individuals with mental health issues. Group psychotherapy should be offered free of charge to individuals with mental health issues by social health providers.