Group cohesion refers to a sense of belonging, mutual support and identification with other group members. Group cohesion has been associated with better outcomes, lower drop-out rates, more interpersonal support and better participation in psychotherapy. Nevertheless, the role of group cohesion in CBT has not yet received much attention. The rationale for delivering CBT in groups is that patients can model themselves through each other due to their similarities in symp-toms. However, there has recently been a shift towards transdiagnostic CBT protocols, in which patients with varied diagnoses participate in the same groups. This shift challenges the rationale of delivering CBT in groups, and it is therefore highly important to understand if and how group cohesion develops in mixed diagnoses CBT groups. The current study used a qualitative comparative framework to investigate the patients’ experiences of group cohesion in diagnosis-specific versus transdiagnostic CBT groups. Twenty-three patients were interviewed with semi-structured in-terviews upon completion of the treatment. Participants had a primary diagnosis of MDD, panic disorder, agoraphobia or social anxiety disorder. A comparative thematic analysis was carried out. Three themes were found: the move from differences to similarities, the role of group cohesion in group CBT and factors helpful and hindering to group cohesion. Group cohesion developed across groups and was considered highly important in both diagnosis-specific and transdiagnostic CBT groups.