Most adults on the autism spectrum have co-occurring mental health conditions, creating a high demand for mental health services -- including psychotherapy -- in autistic adults. However, autistic adults have difficulties accessing mental health services. The most-reported barriers to accessing treatment are therapists' lack of knowledge and expertise surrounding autism, as well as unwillingness to treat autistic individuals. This study was conducted by a participatory autism research group and examined 498 adult-patient psychotherapists on knowledge about autism and self-perceived competency to diagnose and treat autistic patients without intellectual disability compared to patients with other diagnoses. Psychotherapists rated their education about autism in formal training, and competency in the diagnosis and treatment of patients with autism, lowest compared to patients with all other diagnoses surveyed in the study, including those with comparable prevalence rates. Many therapists had misconceptions and outdated beliefs about autism. Few had completed additional training on autism, but the majority were interested in receiving it. Greater knowledge about autism was positively linked to openness to accept autistic patients. The results point to an alarming gap in knowledge necessary for adequate mental health care for individuals with autism.