This study contributes to an operationalization of the concept of action competence for sustainability through the theoretical development and empirical validation of a new 12-item Likert-scale questionnaire: "the Self-Perceived Action Competence for Sustainability Questionnaire," SPACS-Q. Other scales in environmental and sustainability education (ESE) typically measure concepts such as pro-environmental and sustainability attitudes and behaviors, and therefore do not fully cover the concept of action competence for sustainability. An action differs from a 'mere' behavior in that it is voluntary and targeted at bringing about change, which is the overarching goal of ESE. We define action competence as a latent capacity among individuals to act sustainably. We introduce a novel scale measuring this seminal concept. Totally, 614 Swedish adolescents aged 12-19 participated in this study. The scale includes three latent subconstructs: i) knowledge of action possibilities, ii) confidence in one's own influence, and iii) the willingness to act. Confirmatory factor analyses, reliability measures and investigation of convergent validity reveal a questionnaire instrument with excellent psychometric quality. We put forward that the SPACS-Q is a novel and theory-driven, empirically reliable and valid, instrument, and encourage fellow researchers to use the SPACS-Q when investigating people's action competence for sustainability in various contexts.