Little is known about the nature of social anxiety in autistic people: What is similar and different about factors contributing to social anxiety in autistic and non-autistic people? There is also very limited research about autistic people's experiences of therapy for social anxiety in current routine clinical practice. This study investigated these issues by analysing responses to an online questionnaire for 308 adults with elevated social anxiety (232 autistic people and 76 non-autistic people) using mixed-methods approaches. Distinct factors influenced social anxiety in autistic participants, including individual and environmental factors relating to autistic experiences in the social world. We propose a 'distinct' model of social anxiety in autism. In addition, many participants had not had helpful experiences in therapy, especially cognitive behaviour therapy, for social anxiety: They thought therapists did not always acknowledge the basis of their social fears, did not adapt the therapy process in an individualised way and did not understand neurodiversity or recognise neurodivergence. Our results suggest that social anxiety needs to be understood specifically in the context of autism, and if this is neglected, therapy is unlikely to be helpful. Based on participants' comments, this article makes some practical recommendations for therapists supporting autistic adults with social anxiety.