This is a mixed-methods case study aiming to understand teachers' self-efficacy beliefs for sustainability education in the context of climate change education. Therefore, we studied teachers' self-efficacy beliefs from their own perspective as well as the connection between self-efficacy and related concepts, perceived content knowledge (pCK) and perceived pedagogical content knowledge (pPCK). Twenty-two teachers from 18 countries participating on a climate change education professional development camp answered to a questionnaire, and 19 of them were interviewed using semi-structured interviews. The results of this study show that, according to the interviews, the studied teachers have willingness to continuous learn and develop their sustainability teaching. They have beliefs not only about sustainability education but also about themselves as sustainability educators, which is something that the future instruments on teachers' beliefs should acknowledge. The questionnaire results however showed that teachers had the lowest self-efficacy beliefs on supporting students' actions for sustainability. Teachers' pCK and their pPCK were shown to correlate with their self-efficacy, especially with items related to action. In conclusion, teacher education could focus on supporting teachers pCK and pPCK to develop teachers' action-related self-efficacy beliefs thus supporting students to act for sustainability.