The goal of Indigenous education is that it should be approached on the basis of the Indigenous language and culture; this is also the case with Sami education. The Sami School Board has stated that all teaching in Sami schools should be culturally based, despite the fact that Sami culture-based teaching is not specifically defined. Therefore, teachers themselves must adapt the teaching and as a result, usually no Sami culture-based mathematics teaching takes place. The aim of this article is to discuss Indigenous teachers' experiences with designing and implementing culture-based mathematics activities in Sami preschool and primary school. The teachers' work with culture-based mathematics activities took the form of "Sami cultural thematic work with ethnomathematical content," "Multicultural school mathematics with Sami cultural elements," and "Sami intercultural mathematics teaching." Culture-based mathematics activities took place within an action research study in the Swedish part of Sapmi. Sapmi comprises northern Norway, Sweden, and Finland, as well as the Kola Peninsula in Russia. In the action research study, six teachers conducted culture-based mathematics activities in preschool and primary school on the basis of the action research loop "plan-act-observe-reflect." During the study the teachers changed from a problem-focused perspective to a possibility-focused culture-based teaching perspective characterised by a self-empowered Indigenous teacher role, as a result of which they started to act as agents for Indigenous school change. The concept of "decolonisation" was visible in the teachers' narratives. The teachers' newly developed knowledge about the ethnomathematical research field seemed to enhance their work with Indigenous culture-based mathematics teaching.