Autistic adults experience high rates of metal health difficulties and face significant barriers to accessing appropriate mental health care. Empirical research and recent professional guidelines emphasise the importance of modifying standard mental health interventions to best meet the needs of autistic adults. This systematic review explored mental health professionals' experiences of adapting mental health interventions for autistic adults. A systematic search was conducted on CINAHL, PsychINFO, PubMed, Scopus, and Web of Science in July 2022. The findings from 13 identified studies were synthesised using thematic synthesis. Three major analytical themes were generated, the unique experience of adapting interventions for autistic clients, factors which facilitate successful adaptations, and challenges to adapting interventions. Each theme contained a number of subsequent sub-themes. Professionals view the process of adapting interventions to be a highly individualised process. A range of personal traits, professional experiences, and systemic, service-based issues were identified in facilitating or challenging this individualised process. Further research regarding adaptations with different intervention models and increased supportive resources are required to enable professionals to successfully adapt interventions for autistic adult clients.