<p>This study examines the effect of mathematization-based instruction on the epistemological beliefs of pre-service mathematics teachers. To achieve this objective, a mixed-methods intervention design was employed. Data were collected through questionnaires, semi-structured interviews, and classroom observations from 57 pre-service teachers (30 in the comparison group and 27 in the intervention group) drawn from two teacher education colleges in Oromia, Ethiopia. Quantitative data were analyzed using independent samples t-tests and one-way MANOVA, while qualitative data were analyzed thematically. The results of the independent samples t-test indicated that mathematization-based instruction had a significant impact on beliefs concerning the source, stability, and structure of mathematical knowledge. Similarly, the MANOVA results revealed statistically significant mean differences between the intervention group and the comparison group on the combined dimensions of source, stability, and structure of mathematical knowledge. Qualitative findings further showed that the instruction supported pre-service teachers in adopting both absolutist and fallibilist perspectives on mathematical knowledge. Overall, the findings suggest that mathematization-based instruction plays a crucial role in transforming pre-service teachers’ epistemological beliefs, underscoring its importance for teacher educators, curriculum developers, and students.</p>