This study describes the development and psychometric evaluation of a new self-report measure of social cognition: the Theory of Mind Inventory:Self-report-Adult (ToMI:SR-Adult). Adults with autism (or a suspicion of autism; n = 111) and typically developing adults (n = 109) completed a demographic questionnaire and the ToMI:SR-Adult online. Both quantitative and qualitative self-reports of one’s own theory of mind functioning were collected. The ToMI:SR-Adult performed well under all examinations of reliability and validity (internal consistency, accuracy of classification, contrasting-groups). The qualitative data confirmed impressions of validity and revealed that the adults in our sample had high levels of self-insight regarding their own theory of mind. The ToMI:SR-Adult is offered as a promising research and clinical tool for the assessment of social cognition in adults.