<![CDATA[Mathematical reasoning is a fundamental competency in elementary mathematics education because it enables students to understand concepts, construct logical arguments, formulate problem-solving strategies, and draw appropriate conclusions. However, preliminary observations indicate that third-grade students’ mathematical reasoning ability remains relatively low, partly because classroom instruction is dominated by teacher explanations and provides limited opportunities for students to investigate and construct their own understanding. This study aims to examine the effect of the Group Investigation (GI) learning model assisted by Viaran on the mathematical reasoning ability of third-grade elementary school students. A quantitative approach with a quasi-experimental method and a nonequivalent control group design was employed. The participants comprised 62 third-grade students, with 31 students from SD Negeri Klepu 01 assigned to the experimental group and 31 students from SD Negeri Klepu 02 to the control group. Data were collected through mathematical reasoning tests, questionnaires, observations, and documentation and analyzed using descriptive statistics, normality and homogeneity tests, an independent-samples t-test, and simple linear regression. The findings revealed a significant difference in mathematical reasoning ability between the experimental and control groups. The experimental group achieved a higher posttest mean score of 90.19 than the control group, which obtained a mean score of 75.68, with a significance value of p < .001. Regression analysis further demonstrated a significant effect of the Viaran-assisted Group Investigation model on students’ mathematical reasoning ability, with an R² value of .822, indicating that the model explained 82.2% of the variance in mathematical reasoning scores. These findings demonstrate that integrating Group Investigation with Viaran can foster a more active, collaborative, and meaningful mathematics learning environment while strengthening students’ mathematical reasoning. This study contributes empirical evidence supporting the integration of investigation-based learning and interactive instructional media as alternative strategies for developing higher-order mathematical thinking in elementary education.]]>
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