<p>This paper presents the development and psychometric validation of the Adolescent Girls Agency scale (AGAS), a novel instrument specifically designed to measure agentic capacity among girls in East and Southern Africa. Addressing a critical gap in evaluation tools, the AGAS provides a structured framework for assessing girls’ agentic capacity across four distinct domains: self-beliefs, gendered environmental beliefs, self-governance skills, and leadership skills. The validation process employed a rigorous mixed-methods design, integrating qualitative cognitive interviews to ensure cultural relevance and clarity with item response theory analyses to assess psychometric properties, including reliability and validity, and to guide iterative scale refinement. Triangulation of statistical evidence on items requiring further review with stakeholder feedback ensured that the instrument reflected the lived experiences of girls in the target region while aligning with programmatic objectives. The resulting final version of the AGAS provides a robust tool for pre–post evaluation of interventions aimed at strengthening girls’ agentic capacity in East Africa. By offering domain-specific insights into how programs influence different dimensions of agency, the AGAS supports targeted program improvements and more informed decision-making. While three of the AGAS’s four domains are hypothesized to be universal, further research is required to establish reliability and validity across diverse settings to support recommendations for adaptation in other comparable global contexts.</p>