Critical thinking (CT) has been defined as an important goal in education for sustainable development (ESD). In recent curricula, reform for primary education CT has been reinforced as a new key competence. Primary school teachers are struggling to enact CT in their classrooms, and previous research cannot provide the answers to this problem because it has focused on higher education. To implement CT in ESD, first, more must be known about primary school teachers’ understanding and teaching practices of CT. In this study, 10 Norwegian fifth- to seventh-grade teachers were interviewed to capture these aspects. The interview transcripts were subject to thematic analysis. The findings show that the primary school teachers view CT as a cognitive strategy aimed at outlining the multidimensional aspect of ESD, but they leave out many aspects of CT relating to the disciplinary, multidisciplinary, and emotional factors of ESD. Furthermore, they do not teach CT in a way that facilitates the overarching goal of ESD, which is to promote action competence for sustainability. Therefore, suggestions about how to organize teacher professional development are discussed.