Educators continually grapple with the challengeChallenges of seamlessly integrating ScienceScience, TechnologyTechnology, EngineeringEngineering, and MathematicsMathematics (STEMSTEM) into their curriculaCurriculumcurricula, emphasising real-life contextContext, interdisciplinaryInterdisciplinary thinkingThinkinginterdisciplinary, and collaborative inquiryInquiry-based learningLearning to foster twenty-first century skills. This chapter presents the outcomes of a professional developmentProfessional Development initiative aimed at enhancing STEMSTEM integrationIntegration for teachersTeacher in Southeast AsiaSoutheast Asia. The program facilitated collaborative, interdisciplinaryInterdisciplinary teamwork among educators to design STEMSTEM units aligned with a regional MathematicsMathematics and ScienceScience curriculum. The study evaluates the curriculum intentions and interdisciplinaryInterdisciplinary nature of these units, with particular attention to the connections between ScienceScience and MathematicsMathematics. Analysis of the STEMSTEM units revealed a predominant focus on a single scienceScience topic supported by multiple mathematicsMathematics topics. Key mathematicsMathematics areas included Geometry, Space, Measurement, Statistics, Probability, and Data Handling, while prominent scienceScience themes centred on Life and Living, as well as ScienceScience, EngineeringEngineering, and TechnologyTechnology for Sustainable Societies. The explicit articulation of thinking processes, attitudes, and values within the regional curriculum enabled teachersTeacher to effectively integrate these elements into their interdisciplinaryInterdisciplinary STEMSTEM units. The strategic focuses of the units are related to localised environmental and sustainability issues, such as pest management, animal conservation, air and water pollution, and sustainable energy. This study highlights the importance of clear curricular guidelines and collaborative approaches in fostering meaningful interdisciplinaryInterdisciplinary STEMSTEM education.