<p>This study investigated the impact of STEM-integrated instruction on ninth-grade students’ conceptual understanding of pressure in science. An explanatory sequential mixed-methods design was employed, combining quantitative and qualitative approaches to provide a comprehensive account of the outcomes. In the quantitative phase, a quasi-experimental one-group pretest–posttest design was employed with 34 female students from a public school in Karaj, Iran, who were selected through convenience sampling. Statistical analyses using the Wilcoxon signed-rank test revealed a significant increase in mean scores from 11.82 (pretest) to 15.29 (posttest) (<i>p</i> &lt; .001). Effect size indices (<i>r</i> = .87, CLES ≈ 1.00) indicated a large effect. Qualitative findings revealed four interconnected mechanisms through which STEM activities supported students’ conceptual understanding of pressure: emotions and attitudes, learning processes, skills and challenges, and creativity and innovation. Positive emotional engagement reduced anxiety and increased motivation. Hands-on, experiential learning transformed abstract principles (e.g., P = F/A, Pascal’s principle) into concrete, embodied experiences. Collaborative group work promoted confidence, negotiation skills, and conceptual clarification through peer discussion. Design and creativity tasks required students to apply pressure concepts in novel contexts, reinforcing understanding through active problem-solving. Despite challenges such as group coordination, time constraints, and limited resources, the study suggests that integrating STEM into science instruction may enhance not only conceptual learning but also cultivate social, process-oriented, and twenty-first-century skills.</p>