Transition-age youth and young adults with autism spectrum disorder have complex healthcare needs, yet the current healthcare system is not equipped to adequately meet the needs of this growing population. Primary care providers lack training and confidence in caring for youth and young adults with autism spectrum disorder. The current study developed and tested an adaptation of the Extension for Community Healthcare Outcomes model to train and mentor primary care providers (n = 16) in best-practice care for transition-age youth and young adults with autism spectrum disorder. The Extension for Community Healthcare Outcomes Autism Transition program consisted of 12 weekly 1-h sessions connecting primary care providers to an interdisciplinary expert team via multipoint videoconferencing. Sessions included brief didactics, case-based learning, and guided practice. Measures of primary care provider self-efficacy, knowledge, and practice were administered pre- and post-training. Participants demonstrated significant improvements in self-efficacy regarding caring for youth/young adults with autism spectrum disorder and reported high satisfaction and changes in practice as a result of participation. By contrast, no significant improvements in knowledge or perceived barriers were observed. Overall, the results indicate that the model holds promise for improving primary care providers' confidence and interest in working with transition-age youth and young adults with autism spectrum disorder. However, further refinements may be helpful for enhancing scope and impact on practice.