Technology is an indispensable part of the educational curriculum, and large budgets have been allocated to provide technological infrastructure in secondary education institutions in Turkey. It is important that teachers have the technological pedagogical content knowledge (TPCK) to successfully integrate technology into their courses. The aim of this study is to examine the development of the TPCK of a mathematics teacher who participated in in-service training course organized for the use of Geogebra software in geometry teaching. The in-service training is designed in accordance with the Technology Integration Model stages, and Geogebra software is used taking into account the learning outcomes of 9th and 10th grade secondary school geometry. The case study method has been employed, and data was collected through interviews, observation, self-assessment forms, and field notes. The findings show that the in-service training helped the maths teacher integrate technology in the teaching and developed the knowledge of curriculum, students’ understandings and learnings, and instructional strategies and methods. With regard to research recommendations and results, professional development programs that will provide TPCK development can be organized so that teachers can use the ever-evolving technologies in their classes along with the appropriate pedagogical approaches.