Autism significantly impacts how individuals interact, communicate and perceive the world. Gastrointestinal disturbances, particularly constipation, are prevalent among autistic people, affecting their overall well-being. However, to the best of our knowledge, no specific guidelines are currently available regarding the diagnosis of constipation among this population. This systematic review investigates various diagnostic methods employed in studies addressing constipation among autistic adults. It aims to identify discrepancies between the symptoms reported based on diagnostic methods and the actual symptoms experienced by this population. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, nine studies conducted between 2012 and 2022 were included, revealing a predominance of non-prospective designs and an emphasis on children and adolescents. Diagnostic tools varied, including standardized criteria, questionnaires and observation. The prevalence of symptoms not outlined in the official criteria, such as sleep disorders and challenging behaviours, emerged as crucial aspects requiring comprehensive assessment in autistic people experiencing constipation. The study highlights the need for age-specific research in order to develop tailored assessment tools to improve diagnostic accuracy and enhance the overall management of constipation in autistic adults. Recognizing the diverse manifestations of constipation in this population is crucial for developing nuanced interventions and advancing healthcare strategies.