Social identities relate to psychological perceptions of group memberships and form part of the self-concept. Socially identifying with groups has previously been found to associate with better mental well-being outcomes. This study first examined the factor structure and the reliability of measuring social identification in autistic adults. Confirmatory factor analysis showed that a factor structure was replicated in this sample for social identification with other autistic people, but not the family. Second, the study assessed the level to which autistic adults socially identified with different groups, the total number of social identities and whether these were associated with their mental well-being. Autistic adults reported feelings of social identification with many kinds of groups, some with multiple groups, whereas others did not socially identify with any group. Stronger feelings of social identification towards other autistic people and towards one's family, and with more groups overall, were associated with less severe self-reported depression symptoms and more facets of positive mental health. These findings indicate the importance of facilitating autistic people's engagement with social groups.