The ability for a child to engage in the classroom is associated with better academic outcomes. Yet, there is limited information on how child characteristics of autism and adult behavior impact engagement. This study examined (1) the pattern of adult participation and child engagement in preschool classrooms that serve children with ASD, (2) the associations between child engagement and adult participation, and (3) how characteristics of ASD (autism severity, language ability, and challenging behavior) moderate the relationship between adult participation and child engagement. Overall, children were less likely to be engaged when adults were actively or passively participating with them. Moderators impacted this relationship. Children with higher levels of autism severity were more likely to be engaged when adults were actively or passively participating with them. Similarly, children with lower language abilities were more likely to be engaged when adults were actively or passively participating with them. Finally, children with higher levels of challenging behaviors were less likely to be engaged when adults were actively or passively participating with them. These findings have important implications for how adults can best support the engagement of children with ASD.