This study aims to develop a GeoGebra-supported learning kit for teaching polygons and to examine mathematics teachers' opinions about it. The research was designed using a descriptive research model. The study group comprised 11 mathematics teachers who voluntarily participated in the in-service training called 'Dynamic Math Software (Geogebra)' in Turkey. In the study, data were collected through the 'Learning Kit Evaluation Form' and 'Questionnaire'. According to the opinions of the mathematics teachers, the GeoGebra-supported learning kit had enough fidelity related to learning outcomes, instruction, and programming. The opinions of the teachers during their experience in performing the GeoGebra-supported learning kit during in-service training indicated the changing role of the teacher in the classroom, advantages on the cognitive and affective development of the students, the quality of teaching; difficulties and disadvantages of using the GeoGebra-supported learning kit, and their intention to use the GeoGebra-supported learning kit in the future.