Interventions for promoting physical activity designed specifically for preschoolers with autism spectrum disorders (ASD) are lacking, which is problematic given the low rates of physical activity among this population. This study used a stakeholder-engaged approach to adapt the Wellness Enhancing Physical Activity for Young Children (WE PLAY) teacher training for preschoolers with ASD. Specifically, we solicited and integrated feedback from experts in ASD, adapted physical education, and school-based physical activity promotion, and we incorporated behavioural teaching strategies while preserving the intervention's core components and theoretical basis. Following the adaptation process, teachers and a public preschool director implemented WE PLAY-Autism. This case study reports on the adaptation process and process evaluation from the WE PLAY-Autism pilot study. Results indicated high feasibility and acceptability. WE PLAY-Autism warrants further research and dissemination to preschools that aim to appropriately include students with ASD in physical activity.