Argumentation is fundamental and essential in mathematics education. It promotes deep mathematical understanding and helps students connect abstract ideas logically. However, teachers and students in the traditional teacher-centered classroom face difficulties teaching and learning math argumentation. In this article, a cooperative conversation-based tutoring system to foster argumentative skills in learning Pythagorean Theorem was designed. The study analyzed results from an experiment involving 118 middle school students in Taiwan who engaged in formulating, validating, generalizing, and justifying learned mathematics concepts with the help of virtual tutors and peers. The control group included 82 middle schoolers who received the same learning content in the teacher-centered classroom setting. Results confirmed that this agent system promoted significantly better learning outcomes, and the learners' argumentative experience was also enhanced. Moreover, the unique interactions that took place between each student and the agent system resulted in numerous opportunities for learning mathematical argumentation through an adaptive learning mode.