This systematic review aims to offer an updated understanding of the relationship between omega-3 supplementation and/or vitamin D and autism spectrum disorders (ASD). The databases PubMed, Cochrane Library, Web of Science, EMBASE, CINAHL, Vip, CNKI, Wanfang, China Biomedical Database databases were searched using keywords, and relevant literature was hand-searched. Papers (n = 1151) were systematically screened and deemed eligible since 2002. Twenty clinical controlled studies were included in the final review. The findings were analysed for intervention effects focusing on the core symptoms of ASD, included social functioning, behavioural functioning, speech function and biomarkers changes. The review found that the effects of omega-3 supplementation on ASD were too weak to conclude that core symptoms were alleviated. Vitamin D supplementation improved core symptoms, particularly behavioural functioning. omega-3 and vitamin D supplementation combined had a good effect on people with ASD, with significant improvements in social and behavioural outcomes.