This study reports on how context-based STEM (Science, Technology, Engineering, and Mathematics) education can be integrated with Education for Sustainable Development (ESD) in secondary schools through a probiotic food project. Conducted at a public high school in Can Tho City, Viet Nam, the project involved 100 tenth-grade students in designing and producing probiotic products from locally sourced ingredients. The curriculum was structured around a seven-step STEM instructional model emphasizing problem identification, interdisciplinary learning, and community relevance. Quantitative data were collected via a 15-item Likert-scale survey measuring students’ reflections on STEM competencies, sustainability awareness, and engagement. A structured teacher reflection journal provided qualitative insights into pedagogical transformation and implementation challenges. Findings indicated that students developed stronger problem-solving skills, close connections between STEM content and real-world agricultural contexts, and increased interest in sustainability-related careers. The teacher’s reflection revealed shifts from traditional content delivery to facilitation, along with practical constraints related to time and coordination. This study contributes to the growing body of literature on localized STEM-ESD integration and highlights the value of authentic, place-based learning in promoting both scientific and civic competencies among secondary school students.