Background: Comorbid psychopathology and stress were considered possible mediators that may explain the relationship between some autistic traits and repetitive behaviours. The current study sought to examine the mediational effects of comorbid psychopathology, executive dysfunctions and stress in the relationship between some autistic traits and repetitive behaviours. Method: A battery of questionnaires including measures of autistic traits, repetitive behaviours, stress, executive dysfunctions and comorbid psychopathology were administered to a sample of adults with autism and intellectual disabilities (n?=?43). Results: We found that when taken as set dimensions of comorbidity, dysexecutive functioning and stress mediated or explained the effects of autistic symptoms on repetitive behaviour. The total model explained 60% of the variation in repetitive behaviours (R?=?0.60; F?=?13.64, P?<?0.001). The results are discussed in the light of pertinent previous research and their clinical implications, and suggestions for future research are provided. Conclusions: According to the investigated model, increased levels of comorbid psychopathology and stress explained the relationships between repetitive/restrictive behaviours and autistic traits in adults with autism, while executive functioning did not contribute to that relationship.