A broadening of diagnostic criteria and increased awareness of autism has led to a large number of individuals whose difficulties remained undetected until adulthood. This systematic review aimed to synthesise empirical evidence of the experience of individuals who received their diagnosis of autism in adulthood. Eight studies met the inclusion criteria. A meta-ethnographic approach was used to synthesise the findings. Analysis revealed two meta-themes: feeling "like an alien" and the "not guilty" verdict, each with three associated subthemes. The findings indicate the need for timely diagnosis and provision of post-diagnostic support to alleviate the mental health implications of not having a framework to understand one's experiences and to support the process of adjustment to the diagnosis.