Considering the interdisciplinary nature of the sustainable development concept, this study aims to investigate the effect of a cross-curricular mathematical modeling course on pre-service teachers' sustainable development attitudes and mathematical modeling self-efficacies and to investigate their opinions about the course. The sample for this mixed-method study with concurrent triangulation design was 44 pre-service mathematics teachers who were enrolled in a university from the western region of Türkiye. The data for the study is gathered through the sustainable development attitude scale, the mathematical modeling self-efficacy scale, and open-ended questions administered before and after the cross-curricular mathematical modeling course. Findings and analysis of the quantitative and qualitative data revealed that pre-service teachers showed a statistically significant improvement in both sustainable development attitude and mathematical modeling self-efficacy scores. Their statements from open-ended questions also supported the quantitative findings.