Relatively little has been published about the prevalence of autism in adults with psychiatric disorders. In this study, all new patients referred to an adult psychiatric outpatient clinic in Sweden between November 2019 and October 2020 (n = 562) were screened for autism spectrum disorders using the Ritvo Autism and Asperger Diagnostic Scale Screen (RAADS-14). Out of the 304 (58%) responders, 197 who scored above the cut off (14) were invited to participate in an in-depth assessment. Twenty-six of the 48 that participated in the assessment met criteria for ASD and an additional eight had subthreshold ASD symptoms. We estimated the prevalence of ASD in this population to at least 18.9%, with another 5-10% having subthreshold symptoms.