Children with ASD have difficulties socially engaging with adults and peers. The prototypical approach to remediate these difficulties is to directly teach the child appropriate social skills. However, researchers have consistently demonstrated that children do not maintain or generalize these skills when taught in non-naturalistic environments. One way to support the social engagement of children with ASD is to identify features of their natural environment that increase the likelihood of social interaction. This study examined the (1) social engagement of preschoolers with ASD in classrooms with peers and adults, (2) ecological features of preschool classrooms that promoted social engagement, and (3) relationships between social engagement and ecological features. The CASPER-III, an ecobehavioral observational system, was used to code data on the social engagement and classroom ecology of 68 preschoolers with ASD. Children in the study were between 3-5 years of age, enrolled in a public school-based preschool program, and had a clinical or educational diagnosis of developmental delay or ASD. Ecological variables examined included: activity area, child behavior, group arrangement, adult behavior, and initiator of activity. The proportion of social engagement with peers and with adults during each category of ecological variables was calculated, and was compared to the base rate of social engagement with peers and with adults across all ecological features. Overall, children with ASD were more likely to be socially engaged with peers in the Books and Food/Snack classroom areas, when participating in Book or Large Motor behaviors, in Small Groups or Large Groups with an Adult, and during child-initiated activities. Children were more likely to be socially engaged with adults when in the Large Motor and Books areas, when engaged in Book, Preacademic, or Large Motor behaviors, when 1:1 with an Adult, in a Small Group with an Adult or Large Group with an Adult, and when adults showed Approval. Implications for practice and future research are highlighted.