The loss of insect biodiversity is a major global sustainability issue that is highly relevant to science education. Science education can support and develop learners' competence to take actions to sustain insect biodiversity and empower learners to deal actively with this sustainability issue. However, we currently lack an instrument to assess these aspects of individual competence. This paper aims to fill this gap by introducing the Self-Perceived Action Competence for Insect Conservation scale (SPACIC). This scale allows for investigating learners' action competence by focusing on self-perceived knowledge, confidence, and willingness to take insect conservation actions. The scale is grounded in theory and face-validated by external experts. The piloting with 180 secondary school students showed a good quality of the instrument in terms of reliability and validity, as the reliability analyses and confirmatory factor analysis show. The SPACIC scale is applicable to various formal and informal educational settings. Applying the scale can yield information about the effects of educational approaches and inform learners, educators, and researchers about changes in self-perceived competence. In this way, the SPACIC scale can contribute to the evaluation and design of educational approaches and eventually boost learners' development into becoming active environmental citizens.