Action competence consists of the knowledge, willingness, and self-efficacy for contributing to a controversial issue such as sustainable development. As such, action competence in sustainable development (ACiSD) is a desired outcome of education for sustainable development (ESD). Still, the scarce instruments for measuring ACiSD that have been developed to date, are not specifically designed for early adolescence, when civic involvement is developed. Therefore, this study reports on the development of such a measurement instrument: the Action Competence in Sustainable Development Questionnaire (ACiSD-Q). A mixed-method approach in four steps used three different samples: after a literature review (step 1), early adolescents (n = 75) informed the generation of an initial item pool (step 2; qualitative). After assessment of the scale's content validity it was administered to a second sample (n = 403) to test psychometric properties (step 3; quantitative). Finally, rigorous statistical analyses (third sample, n = 1796) confirmed the proposed structure, reliability, construct, and predictive validity of the final ACiSD-Q (step 4; quantitative). Our findings support a valid and reliable third-order model, fit for monitoring ESD efforts that aim to enhance early adolescents' action competence in sustainable development.