This paper explores how children engaged in playing Minecraft in an afterschool program develop mathematical approaches in their in-game activities. The investigation is framed as ethnomathematical in the sense that, rather than searching for specific curricular concepts, it explores the problem situations and explanatory systems that children develop. Aesthetics, symmetry, collaboration, copying, and efficient building strategies all lead to local problem-solving and explanatory systems and can therefore be characterised as steps towards ethnomathematics. In the explored example, collaboration between the children and the afterschool program’s attitude towards children’s collaborative gaming are crucial factors in the way Minecraft supports the development of mathematical thinking.