Spatial ability is a fundamental component in geometry learning and a critical skill for prospective mathematics teachers. However, traditional instructional methods often fall short in fostering students’ spatial reasoning, especially in higher education contexts. This study aimed to investigate the effect of an instructional model integrating Project-Based Learning (PjBL) with ethnomathematics on the spatial ability of pre-service mathematics teachers. Employing a quasi-experimental one-group pretest–posttest design, the study involved 30 participants who received geometry instruction through PjBL with cultural integration. The findings revealed a substantial improvement in students’ spatial ability, indicating that the intervention was both effective and meaningful. The integration of culturally grounded projects enhanced students’ visualization, mental rotation, and engagement. These results support the use of ethnomathematics-based PjBL as an innovative and contextually relevant approach to improving spatial reasoning in mathematics teacher education. Future research is recommended to investigate the long-term effects and adaptability of this model in various educational settings.