Tackling climate change is the goal of the United Nations’ 2030 Agenda for Sustainable Development, which includes seventeen Sustainable Development Goals (SDGs). The current Finnish core curriculum for basic education places importance on strengthening the role of basic education in building a sustainable future. The data comprise responses from 48 pre-service language teachers to an online survey. Their responses to open-ended questions were analysed qualitatively using content analysis and categorised into three categories: (1) defining multilingualism, (2) multilingualism as an action, and (3) language relating to multilingualism. While most participants considered multilingualism to be an individual characteristic, more than half described a multilingual person as someone who knows at least two languages, and two-thirds claimed that multilingualism involves the frequent and fluent use of multiple languages in everyday life, both actively and officially. Overall, the participants were aware of many forms of multilingualism, which equipped them to support their students through multilingual pedagogies and thus promote the sustainability goals set by the UN. However, more research is needed to examine the actual multilingual practices used in the classroom.