STEM is a learning approach that can enhance students' mathematical skills by linking mathematics with science, technology, and engineering, thereby increasing students’ motivation. This study analyzed teachers' understanding and the challenges they face in implementing STEM-based mathematics instruction. A phenomenology-informed qualitative study explored the experiences of 12 mathematics teachers across six provinces in Indonesia: West Java, Central Java, East Java, Yogyakarta, West Sumatra, and West Nusa Tenggara. Data were collected through online interviews and analyzed using the Bogdan & Biklen technique, including data reduction, axial coding, theme determination, and conclusion and interpretation. The results indicate that teachers view STEM-based mathematics as a means to connect subjects, develop critical 21st-century skills, and apply knowledge in real-life contexts. They implement STEM through project-based, interdisciplinary lessons that increase student engagement, despite challenges such as time constraints, curriculum demands, and differences in student readiness. Effective STEM instruction relies on professional development, school support, and sufficient resources to enable teachers to integrate STEM concepts confidently into mathematics teaching. These findings highlight the importance of providing structured guidance, adequate teaching materials, and policy support to enhance teachers’ capacity to deliver STEM-based mathematics effectively, bridging theory and practice for meaningful student learning.