Our objective was to develop a measure of healthcare providers' self-efficacy in providing healthcare to autistic adults and to better understand their training needs. We used a community-based participatory research approach with academic researchers, autistic adults, supporters, and healthcare providers. We developed a one-page questionnaire which included the new 6-item self-efficacy scale, two items on how challenging and rewarding it is to provide care to autistic adults, and seven items on provider characteristics. We surveyed 143 healthcare providers from eight primary care clinics in Oregon and California, United States. Preliminary psychometric testing found the Academic Autism Spectrum Partnership in Research and Education (AASPIRE) Adult Autism Healthcare Provider Self-Efficacy Scale to have good internal consistency reliability (Cronbach's alpha 0.87) and consist of a single factor. A priori hypothesis testing found correlations in the expected directions. Only a minority of providers reported high confidence in communicating with patients (25%); performing physical exams or procedures (43%); accurately diagnosing and treating other medical issues (40%); helping patients stay calm and comfortable during visits (38%); identifying accommodation needs (14%); and making necessary accommodations (16%). While providers need training across all aspects of care related to autism in adulthood, interventions should pay particular attention to helping providers communicate with patients and identify and make accommodations.