This study aims to investigate the effects of an integrated Science, Technology, Engineering, and Mathematics (STEM) unit designed around the issue of sulfide mining on middle school students' socioscientific reasoning competencies (i.e., complexity, perspective taking, inquiry, skepticism, affordances of science). More specifically, students' level and extent of socioscientific reasoning were measured before and after an integrated STEM unit in the context of sulfide mining. The implemented unit was co-designed with a group of middle school teachers from different disciplines as part of a nine-week professional development program. Data were collected from seventh-grade students by utilizing a revised version of Quantitative Assessment of Socioscientific Reasoning (QuASSR). Students were presented with a scenario followed by five questions associated with socioscientific reasoning dimensions. Data were analyzed by using a rubric on a 0-4 scale. We found that students made improvements in their reasoning associated with complexity, perspective taking and affordances of science, however, no statistically significant increase was found in inquiry and skepticism dimensions. Quantitative analysis showed that students scored highest on perspective taking and lowest on inquiry dimensions after the unit implementation. Qualitative analysis further revealed that students could use a variety of sources when they reason about sulfide mining and exhibited different levels of reasoning in different SSR dimensions. Study findings have important implications for the development of socioscientific reasoning competencies through integrated STEM approaches.