<![CDATA[Mathematical reasoning is essential for understanding concepts, constructing arguments, and solving mathematical problems, yet classroom observations indicate that many students remain passive and have difficulty expressing conjectures, performing mathematical manipulations, and drawing conclusions. Learning activities that require students to formulate and solve their own problems may provide more opportunities to develop these abilities. This study aimed to examine the effect of the problem-posing learning model on students' mathematical reasoning ability. The population comprised all 263 eighth-grade students of SMP Negeri 29 Bandar Lampung in the 2024/2025 academic year, distributed across nine classes (VIII.1–VIII.9). Two classes taught by the same teacher were selected through cluster random sampling. Class VIII.4, consisting of 28 students, served as the experimental group and received problem-posing instruction, whereas class VIII.2, consisting of 30 students, served as the control group and received conventional instruction. The study used a pretest–posttest control-group design. Data were collected through essay tests developed to measure four indicators of mathematical reasoning: presenting mathematical statements, proposing conjectures, performing mathematical manipulations, and drawing conclusions. Students' gain scores were analyzed using the Mann–Whitney U test. The results showed that the mean gain in mathematical reasoning scores of students taught through problem posing was higher than that of students taught conventionally. The experimental group also demonstrated stronger achievement across the measured reasoning indicators. These findings indicate that the problem-posing learning model affects students' mathematical reasoning ability. Requiring students to formulate questions, discuss possible strategies, solve the problems they create, and evaluate their solutions can promote active participation and provide meaningful practice in mathematical reasoning.]]>
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