BACKGROUND: Concerns over self-diagnosis of mental disorders among adolescents have been increasing, but very little empirical research exists to understand what clinicians think about this phenomenon or how it might impact psychological therapy. METHODS: Semi-structured one-to-one interviews were conducted with 16 mental health professionals, exploring their experiences with and attitudes towards self-diagnosis of mental disorders in adolescents. Reflexive thematic analysis was used to analyse the data. RESULTS: Five themes were generated. The first two focused on factors contributing to the rise in self-diagnosis: (1) Self-diagnosis stems from an inherent need to be understood and (2) Access to diagnostic language has proliferated. The latter three focused on the impact of self-diagnosis on therapy: (3) Self-diagnosis is meaningful therapeutic material; (4) When adolescents feel validated, self-diagnosis often fades over time; and (5) Self-diagnosis becomes a barrier to change when it is inflexible. CONCLUSION: The study indicates that, in most cases, clinicians do not consider self-diagnosis as problematic and report it can sometimes help understand the adolescent's concerns. However, our study also supports concerns about harm to the individual when self-diagnosis is inflexible, and how self-diagnosis could be harmful on a population level if it trivialises psychiatric language and suffering. These findings suggest the need for careful management of self-diagnosis in clinical settings and addressing public discourse that promotes self-diagnosis.