<p>In the context of educational reform in Vietnam, developing the competence to design Science, Technology, Engineering, and Mathematics (STEM) curriculum for preservice teachers is a strategic imperative; however, a significant gap between theoretical knowledge and practical skills persists. This study was conducted to design, implement, and evaluate the effectiveness of a pedagogical intervention based on Project Based Learning (PBL) to foster this core competence for preservice teachers in the Natural Science Education program. The study employed a mixed-methods approach with an explanatory sequential design, beginning with a cross-sectional survey to assess the current situation (<i>n</i> = 271), followed by a one group pre-test/post-test pre-experimental study (<i>n</i> = 50) conducted with third-year students at Thai Nguyen University of Education over a 15 week course. Students’ design competence was measured by the scores of their lesson plans in the pre-test and post-test, using a detailed, validated Rubric. The results revealed a profound and highly statistically significant transformation in students’ competence, with the mean score soaring from 18.05 to 29.33 (<i>p</i> &lt; 0.001). The intervention generated an extremely large effect size (Cohen’s d = -5.758), affirming its strong practical impact. Progress was noted uniformly across all five competence domains, particularly in interdisciplinary integration and project process structuring. The study concludes that PBL is not only a suitable method but also a remarkably effective pedagogical pathway to transform theoretical knowledge into practical competence, providing a validated and scalable model for teacher education institutions.</p>