More adults are undergoing autism assessment due to recent changes in awareness, diagnostic criteria and professional practices. This scoping review aimed to summarise research on autism diagnosis in adulthood and identify any gaps for future study. The authors searched for studies involving first-time assessment and diagnosis of autism in adults, which identified 82 studies from 13 countries using various methodologies. Six themes of (1) prevalence, (2) diagnostic pathways and processes, (3) gender, (4) psychosocial characteristics, (5) co-occurring conditions and (6) experiences of diagnosis were identified across the studies. Findings suggest that receiving an autism diagnosis has a significant emotional impact on adults, but accessibility and processes are inconsistent, and formal support services are lacking. More research is needed on autism diagnosis in adults with intellectual disability, consequences of the timing of diagnosis, and support after diagnosis.