This study was conducted to examine the effect of mathematical modelling activities on the mathematical modelling skills of secondary school students in the context of STEM education. The study was designed according to the embedded design, one of the mixed research methods. The study group of research consists of 66 eighth-grade students studying in a public school in the central district of a large province in the south of Turkey in the 2020-2021 academic year. While the criterion sampling method, one of the purposeful sampling methods, was used to determine the quantitative study group of the research, the maximum variation sampling method was used to determine the qualitative study group. On the other hand, in the context of STEM education, mathematical modelling problems, evaluation rubric and semi-structured interview forms were used as data collection tools in the research. As a result of the research; It was concluded that mathematical modelling activities in the context of STEM education positively improved the mathematical modelling skills of secondary school students. In addition, it has been concluded that the students who receive education with mathematical modelling activities applied in the context of STEM education gain different interdisciplinary perspectives, experience positive developments in their thinking skills, adapt to group work more easily, and increase their interest in engineering and technology.