The last decade has experienced a surge of interventions focused on improving adolescent girls' agency. Yet measuring adolescent girls' agency continues to be a challenge, limiting the ability to track impact. This study addresses this evidence gap by constructing and validating a multidimensional measure of agency among adolescent girls in Ethiopia. This study utilized cross-sectional data from 3033 in school adolescent girls aged 10-12 years and 15-17 years and their adult female caregivers collected as part of the Gender and Adolescent: Global Evidence study in 2017-2018 in Ethiopia. This study constructed a measure of agency among the sample and evaluated both known group and convergent validity of the scale. Twenty-two indicators across three domains (decision-making, voice, and mobility) were used to characterize adolescent girls' agency. The data was randomly divided into two halves for exploratory and confirmatory factor analyses separately. While six factors of agency emerged from the initial exploratory analysis, two factors (decision-making and mobility factors) defined adolescent girls' agency from the confirmatory factor analysis. Known-groups validity of the agency scale was confirmed-the scores on the two domains (decision-making and mobility) and the overall agency scale was higher for older girls compared to younger girls and for girls in urban households compared to those in rural households. Convergent validity of the scale was not confirmed. This study advances adolescent girls' agency measurement by providing a validated multidimensional measure that can be used to support future policy and research.