We developed and validated an instrument, the Teacher Self-efficacy for Teaching Students with ASD Inclusive Early Childhood Classrooms Scale: TSE-ASDI/EC, to determine teachers’ self-efficacy for teaching students with autism spectrum disorder (ASD) in inclusive early childhood classrooms. We engaged in literature and expert reviews followed by cognitive pre-testing with pre-service and in-service early childhood teachers. Next, we employed a series of analytic techniques to examine the factor structure, reliability, and relationships among the dimensions of the instrument for the entire sample and by subgroups. We provide evidence for validity based on test content, response process, internal structure of the instrument, and relations to other variables. This led to a unidimensional, 16-item scale to measure general education teachers’ self-efficacy for educating students with ASD in inclusive early childhood classrooms. Based on these findings we discuss implications for research and practice.