The purpose of this study is to examine the effect of a technology-enhanced collaborative learning environment on secondary school students' mathematical reasoning in the concept of triangle. The participants of the study are 30 secondary school students. This study was carried out with the embedded design, one of the mixed methods designs. The quantitative aspect of the study was carried out with the quasi-experimental design including comparison group design. While the experimental group received training in a technology-enhanced collaborative learning environment, the control group students continued their education in a traditional informal-collaborative learning environment. The qualitative aspect of the study included data belonging to a group of four students chosen among the experimental group. The data of the research comprised students' audio and video recordings, screenshots, dynamic mathematics software GeoGebra files, and written products. The quantitative data were collected via open-ended questions including ten items with the intention of revealing mathematical reasoning and the qualitative data were gathered with the designed mathematical tasks. While independent t-test was used to analyse the quantitative data, the qualitative data were analysed with Toulmin's model and dialogical approach. As a result of data analysis, it was found that technology-enhanced collaborative learning environment has a positive effect on students' mathematical reasoning in the concept of triangle.