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. Lay abstract More adults are getting assessed for possible autism. Here, we give an overview on what is already known about autism diagnosis in adulthood and find areas that need more research. We divided results from the studies we found into six topics of (1) rates of autism in different groups; (2) the process of getting an autism diagnosis in adulthood; (3) gender; (4) personality traits, abilities and behaviours of diagnosed adults; (5) mental and physical health conditions that occur together with autism; and (6) how adults think and feel about being assessed and diagnosed. We found that adults often have strong emotions after being diagnosed, the process of getting a diagnosis can be unclear and different for everyone, and not many support services are available for adults. More research on diagnosing adults with intellectual disability, differences between early and late-diagnosed adults, and support after diagnosis would be useful.