This research aims to develop a professional development model for science teachers using a school-based approach to enhance STEM literacy. The model leverages outdoor STEM education resources in southern Thailand. The study focuses on primary-level science and project-based teachers in Phuket Province, selected through purposive sampling from schools participating in an educational sandbox project with support from school administrators. The research employed a mixed-methods approach to design a professional development program emphasizing outdoor STEM activities and incorporating local Phuket contexts. Data collection involved STEM literacy assessments, lesson plan evaluations, classroom observations, and focus group interviews. Quantitative data were analyzed statistically, while qualitative data underwent content analysis over a one-year period. The project began in 2023 with a needs assessment, tool development, and a draft model, followed by implementation in 2024. Findings highlight that school-based coaching and ongoing lesson design workshops, core elements of the program, improved teachers' understanding of STEM integration. Teachers designed learning activities rooted in local contexts and engineering design principles, fostering interdisciplinary connections within a STEM framework. This approach enabled them to integrate STEM concepts into primary education while emphasizing continuous assessment of student outcomes and competencies. Despite the program's success, challenges remain, particularly in allocating time for curriculum implementation and securing stronger administrative support. Nevertheless, the program advanced STEM literacy by using local contexts as a platform, aligning with community-based education goals and enhancing primary-level STEM education practices.