Argumentation is an important part of education in different disciplines such as science teaching and psychology. Numerous research studies have revealed that the use of argumentation practices in courses have a positive impact on students' both cognitive (critical thinking, reasoning, etc.) and affective abilities. Recently, the number of studies on argumentation in the field of mathematics education has increased. It is expected that prospective teachers today are key elements of the argumentation process as they can encourage students to produce strong mathematical arguments; hence, they themselves are also expected to develop their own argumentation skills. In the literature, there is a need for studies on the reasoning processes and argumentation structures of prospective teachers. Thus, the present study aimed to investigate the global argumentation structures used by prospective middle school mathematics teachers while they were solving geometry tasks. In line with this purpose, a qualitative case study was conducted with 8 prospective teachers who were enrolled in the elementary mathematics education department of a state university located in Ankara. Two argumentation applications were conducted with the participants. In the first application, two problems on triangles and in the second one two problems on circles were solved by the participants. Then, the global argumentation structures of the participants were examined. The results showed that mathematical reasoning of prospective middle school mathematics teachers was weak and, therefore, they mostly used simple global argumentation structures. On the other hand, it was found that the researcher's directions lead participants to use more complex global argumentation structures. Therefore, it was concluded that the teachers who orchestrate the argumentation process should have sufficient knowledge and experience regarding argumentation. This result revealed the importance of focusing on the development of mathematical reasoning skills of prospective teachers. Based on this, it is recommended that necessary adjustments be made in teacher training programs to include argumentation so that prospective teachers can be equipped with the ability to orchestrate argumentation in their lessons.