This article reports on a cross-sectional study tracing possible changes in primary teachers' self-efficacy in teaching mathematics across various points in their professional careers, involving both novice and experienced pre-service teachers (PSTs) and in-service teachers (ISTs). In the relevant literature, self-efficacy appears to be operationalised differently for PSTs and ISTs. Here, we conceptualise it as a unidimensional construct when measuring the future-oriented beliefs teachers hold regarding their own ability to explain mathematics to others, with a focus on understanding the underlying mathematical concepts and procedures. To measure teacher self-efficacy, we used a previously developed and validated 20-item instrument. Participants included novice PSTs (n = 191), experienced PSTs (n = 130), novice ISTs (n = 119) and experienced ISTs (n = 194). Rasch analysis enabled a comparison of the results from the four groups, confirming the theory-based expectation that self-efficacy in teaching mathematics develops with experience. The article concludes with a discussion of the implications of our work, as well as directions for further research. For example, future studies could shed light on the conditions under which the development of self-efficacy can be supported more effectively at different transition points of mathematics teachers' professional careers.