INTRODUCTION: Early childhood education and intervention programmes can improve the developmental outcomes for priority groups of children. However, in Australia, a culturally responsive developmental outcome measure that has been validated for use with Aboriginal and Torres Strait Islander children is required to effectively evaluate impact.The Ages and Stages Questionnaire-Steps for Measuring Aboriginal Child Development (ASQ-STEPS) has been developed to fill this gap. A culturally responsive developmental outcome measure for children aged one to 60 months, the ASQ-STEPS is administered by interview and includes 52-63 items in each of five domains. This study aims to examine the reliability, validity, fidelity and acceptability of the ASQ-STEPS when used with Aboriginal and Torres Strait Islander children. METHODS AND ANALYSIS: This study uses a participatory approach, establishing an Indigenous Reference Group (IRG) and partnering with Aboriginal Community Controlled and government organisations. Up to 250 eligible children and their families will be recruited in urban, regional or remote locations in the Northern Territory, South Australia, Queensland, New South Wales and Victoria. Health or education practitioners will be engaged to administer the ASQ-STEPS. Demographic information, observational data during administrations, and caregivers' and practitioners' perspectives will also be collected.Different analytical approaches will be employed to determine validity, fidelity and acceptability. A unidimensional item response model (IRM) will be fitted to the data by marginal maximum likelihood. Classical and IRM fit statistics and differential item functioning will be employed to examine validity. Internal consistency will provide an index of reliability and measurement error will assess the uncertainty associated with measures of individual children. Fidelity of administration will be observed and described, and acceptability and utility for caregivers and practitioners will be explored through thematic analysis. ETHICS AND DISSEMINATION: This study received ethical approval from the University of Melbourne and the ethical body in each site. Knowledge translation will be guided by the IRG. Results will be disseminated nationally and internationally.