Sustainability education (SE) is included in school curricula to integrate the principles, values, and practices of sustainable development (SD) into all education. This study investigates lower secondary school subject teachers as educators for sustainability. A survey was used to study the perceptions of 442 subject teachers from 49 schools in Finland. There were significant differences between the subject teachers' perceptions of their SE competence, and the frequency with which they used different dimensions of SE (ecological, economic, social, well-being, cultural) in their teaching varied. Teachers' age had a small effect, but gender, school, and its residential location were nonsignificant factors. Teachers could be roughly classified into three different subgroups according to their perceptions of the role of SE in their teaching; those who considered three SE dimensions rather often and used holistic sustainability approaches in their teaching (biology, geography, history); those who considered two or three dimensions often but were not active in holistic teaching (mother tongue, religion, visual arts, crafts, music, physical and health education, and home economics) and those who used one SE dimension or consider only one holistic approach in their teaching (mathematics, physics, chemistry and language). Subject teachers' awareness of their SE competence is important to encourage them to plan and implement discipline-based and interdisciplinary SE in their teaching. The specific SE expertise of subject teachers should be taken into account in teacher training and education.