Recently, researchers have been interested in how autistic people experience loneliness. Yet, most of this research has focused on loneliness in autistic children and young people. We present the results of a systematic review on loneliness in autistic adults. A systematic review is a rigorous way of searching for all existing research on a topic and summarizing the findings about specific questions. We searched for all research published on this topic until 9 April 2021. We found 34 articles that investigated loneliness in autistic adults. This research showed that (1) there is fairly little research that has involved directly asking autistic adults about their first-hand experiences of loneliness (e.g. what loneliness feels like for them); (2) few research studies have used loneliness questionnaires specifically developed for autistic adults (this was attempted in just one research study); (3) collective loneliness (i.e. loneliness associated with how much an autistic person feels they 'fit in' to society) seems important to autistic adults but has not been investigated as commonly as other aspects of loneliness (e.g. loneliness associated with romantic relationships or friendships); (4) things that might increase loneliness in autistic adults include anxiety and depression, and a lack of autism understanding and acceptance, for example; and (5) things that might reduce loneliness in autistic adults include having relationships and self-acceptance, for example. In our article, we discuss the kinds of future research on loneliness in autistic adults that might be useful.