We investigated Autism social identity and mental health in autistic people. Autistic people have social and communication deficits, and experience social stigma—factors that could interfere with the development of positive social identity. Indeed, autistic participants (N = 272) had significantly lower personal self‐esteem, and higher levels of depression and anxiety than typically developing controls (N = 267). Autism social identification was positively associated with personal self‐esteem, and this relationship was mediated by collective self‐esteem (perceived positivity of Autism identity). Furthermore, there were significant negative indirect effects between Autism identification and anxiety, and between Autism identification and depression, through increases in collective self‐esteem and personal self‐esteem. Thus, while autistic participants reported poorer mental health than average, having a positive Autism social identity appeared to offer a protective mechanism. This implies that to improve mental health in the Autism population, clinical approaches should aim to facilitate development of positive Autism identities.