The present observational cross-sectional study aimed at investigating the prevalence of feeding and eating disorders (FEEDs) in adults with intellectual disability (ID)/intellectual developmental disorder (IDD) with or without autism spectrum disorder (ASD) and specific problem behaviours (PBs). Two hundred six adults with ID/IDD consecutively attending residential and rehabilitative facilities, 59.2% of which had co-occurring ASD, were assessed for presence of FEEDs by a structured interview specifically developed for the study and Diagnostic Manual - Intellectual Disability criteria. The 4.3% of the sample fully met the diagnostic criteria for anorexia nervosa, 6.7% for bulimia nervosa (BN) and 22.8% for binge eating disorder (BED). Furthermore, at least one observable symptom of these disorders was found in higher percentages of the sample. A higher prevalence of FEEDs was found in ID/IDD plus ASD than in ID/IDD alone. PBs were also significantly higher in participants with co-occurring ASD and had a positive correlation with the number of FEED symptoms, especially for BN and BED. The study enriches previous literature and considers novel aspects such as the behavioural/observable presentation of symptoms as well as the association with ASD and PBs. These issues deserve a specific consideration within standard psychiatric assessment and future research, especially in persons with major communication and/or cognitive difficulties.