There is a growing interest in exploring the structure of student academic writing across different disciplinary backgrounds, including the Science, Technology, Engineering, and Mathematics (STEM) field. However, despite the availability of relevant literature on STEM student writing, research on the structure of STEM students’ research introductions, particularly within the secondary education context, remains essentially scant. Consequently, STEM student research writers should be redirected towards a genre-based academic writing practice to meet the rhetorical demands of their discourse community. Drawing on this research gap, this qualitative genre analytic study was conducted to explore the structure of STEM students’ research introductions, with an emphasis on the macrostructures and the move/step occurrences. Ten research introductions submitted as preliminary examination papers by the Grade 11 STEM students in an online research writing course at a private Philippine university were collected and further screened via Turnitin, ensuring their authenticity. Following Biber et al.’s (2007) top-down corpus-based discourse analytic framework, moves and steps in the research introductions were carefully analyzed, with Swales’ (1990, 2004) Creating A Research Space (CARS) model as basis for move/step identification. The findings indicated variations in the move structure of students’ research introductions, with the majority deviating from Swales’ (1990, 2004) model. While the students employed all three moves by Swales (1990, 2004), they hardly established a niche in writing a research introduction. The study highlights important implications for pedagogy, curriculum, and teacher professional development in the context of STEM research writing.