This systematic review aimed to evaluate the efficacy of psychological therapies for anxiety for people with autism and co-occurring intellectual developmental disorder (AUT + IDD). A systematic search identified 13 studies comprising 49 participants with AUT + IDD, aged between 5 and 41 years. Most studies were single-case experimental designs (n = 7) or case studies or case series (n = 4). Studies implemented cognitive behavioural therapy (n = 4) or exposure therapy techniques (n = 9). All studies reported a reduction in anxiety symptoms, as measured by either quantitative measures or defined as participants meeting end of treatment criterion. However, the conclusions are preliminary due to the methodological limitations of the current literature. The implications of these findings, as well as recommendations for future direction in the field, are discussed.