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This study aimed to explore student argumentation in a mathematics classroom using lesson study and an open approach. The research methodology employed a qualitative approach that focused on the protocols of student problem solving and the results of the study are presented as an analytical description. The target group was 14 third grade students, divided into four groups in the 2016 academic year. This research collected data while teaching with a lesson study and open approach, analyzing student argumentation. The results revealed that the students who attended mathematics classes with a lesson study and open approach went through the four steps of the argumentation process, which occurred in the second step of the open approach, promoting student self-learning.
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