OBJECTIVE: Autism Behavior Inventory (ABI) is a new measure for assessing changes in core and associated symptoms of autism spectrum disorder (ASD) in participants (ages: 3 years-adulthood) diagnosed with ASD. It is a web-based tool with five domains (two ASD core domains: social communication, restrictive and repetitive behaviors; three associated domains: mental health, self-regulation, and challenging behavior). This study describes design, development, and initial psychometric properties of the ABI. METHODS: ABI items were generated following review of existing measures and inputs from expert clinicians. Initial ABI scale contained 161 items that were reduced to fit a factor analytic model, retaining items of adequate reliability. Two versions of the scale, ABI-full (ABI-F; 93 items) and ABI-short version (ABI-S; 36 items), were developed and evaluated for psychometric properties, including validity comparisons with commonly used measures. Both scales were administered to parents and healthcare professionals (HCPs) involved with study participants. RESULTS: Test-retest reliability (intraclass correlation coefficient [ICC] = 0.79) for parent ratings on ABI was robust and compared favorably to existing scales. Test-retest correlations for HCP ratings were generally lower versus parent ratings. ABI core domains and comparison measures strongly correlated (r ≥ 0.70), demonstrating good concurrent validity. CONCLUSIONS: Overall, ABI demonstrates promise as a tool for measuring change in core symptoms of autism in ASD clinical studies, with further validation required.