Objective: The negative impact of caring for a child with autism spectrum disorder (ASD) on parents’ psychophysiological functioning has been widely evidenced. However, siblings, who also face emotional, social and physical challenges associated with having a brother/sister with ASD, have been less widely studied. This study examined the psychophysiological impact of childhood ASD on siblings. Methods: A sample of 25 siblings of children with ASD (and their mothers) and a control group of 20 siblings of neuro-typical children (and their mothers) completed questionnaires assessing: (a) demographic and lifestyle information, (b) family characteristics, (c) child behaviour problems, (d) social support and (e) depressive symptomology. Saliva samples were collected at several time points on two consecutive days, and estimates of the cortisol awakening response (CAR), diurnal cortisol slope and mean diurnal cortisol output were derived. Results: Total depressive symptoms were higher in siblings of children with ASD compared with controls. Group differences with respect to depressive symptomology were driven more by emotional than functional problems. With respect to physiological functioning, groups were comparable on all cortisol indices. In siblings of children with ASD, social support, especially from parents and close friends, predicted total depressive symptoms, as did the behaviour problems of their brother/sister with ASD. Conclusion: Siblings of children with ASD experience greater emotional problems and overall depressive symptoms compared with a control group. Interventions that enhance social support, as well as helping siblings better understand the behaviour problems of their brother/sister with ASD, might be effective for alleviating depressive symptoms.