The aim of this systematic review was to investigate what is currently known about autistic adults' participation in physical activity and sedentary behaviour and explore any associated correlates of these behaviours. The databases EMBASE, MEDLINE, PsycINFO, PubMed, and SPORTDiscus were searched for relevant literature. The review was registered with PROSPERO (No. 189187). The search identified 2834 papers, with 12 meeting the criteria for inclusion. Correlates of physical activity and/or sedentary behaviour were reported in eight included studies. The limited evidence identified suggest that autistic adults are unlikely to meet Australia's physical activity guidelines. Future research should use device-derived measurement to more accurately quantify activity levels within the autistic adult population.