Abstract We examined five-year-olds’ ability to co-represent a partner's role during a shared activity. In adults, one indicator of such co-representation is the joint Simon effect, a spatial compatibility effect that is present when a two-choice reaction time task (a Simon task) is divided between two participants but is not present when an individual carries performs the task in isolation. We provide evidence for a joint Simon effect in five-year-old children, although its magnitude was unaffected by the priming of interdependence or independence in a preceding activity. Appearance of the joint Simon effect in young children suggests that when carrying out a joint task, young children can form integrated task representations involving both their own role and their partner's role, thus serving as a potential cognitive mechanism that may facilitate the emergence of early joint action abilities.