In this paper, we explore how students' engagement varies in different STEM (Science, Technology, Engineering, Mathematics) learning environments. More specifically, we focus on the significance of a learning environment applying an integrated STEM (iSTEM) approach and the significance of STEM learning environments' student-centredness. Moreover, we explore the relative importance of different student-centred principles (lesson plan and implementation, communicative interactions, student-teacher relationships) for students' engagement in the STEM learning environment. Applying a mixed-method approach, we draw from observational data of 24 STEM lessons in combination with data from seven focus groups with 67 grade 9 students. The quantitative findings, based on the observational data, show that a learning environment applying an iSTEM approach seems to support students' engagement. Further investigation made it clear that the student-centredness in this learning environment is especially significant. Regarding the specific student-centred principles, all principles had a significant impact on students' engagement. The focus group data make clear that, besides student-centredness, the integrative aspect and the use of authentic real-world problems in iSTEM can also be engaging for students. These results indicate that iSTEM is a good practice to engage students in the STEM learning environment, as it facilitates teachers' implementation of a general student-centred approach.