The transition to postsecondary education is characterized by many changes for adolescents with autism spectrum disorder (ASD). A data-based understanding of a student’s readiness for postsecondary education could help students, and their parents, better prepare for this life transition. The Transition Readiness Scale (TRS) was created to address this need. The TRS is a self/other-report questionnaire used to assess postsecondary readiness across behavioral, cognitive, and emotional domains among adolescents 15–18 years of age. The present study details measure development and provides preliminary psychometric properties in a sample of transition-aged youth with ASD. Results indicate strong internal consistency, adequate item-level analyses, and discriminant and concurrent validity. Future validation of the TRS in large-scale field testing is merited to inform clinical interpretation.