Computational thinking (CT) is an important skill to develop in elementary mathematics learning, particularly in plane geometry, which requires conceptual understanding rather than mere memorization of formulas. This study examined the effect of an unplugged CT-based learning approach on fifth-grade students’ mathematics learning outcomes in plane geometry in a micro-sized classroom. A quantitative approach using a one-group pretest–posttest design was employed with seven fifth-grade students at SD Negeri 2 Jatigreges, selected through total sampling. The learning activities incorporated four CT components—decomposition, pattern recognition, abstraction, and algorithms—through unplugged activities without computers. Data were analyzed descriptively and inferentially using a paired-samples t-test in JASP after the difference scores were found to meet the normality assumption based on the Shapiro–Wilk test. The mean score increased from 67.71 on the pretest to 82.00 on the posttest. The test yielded t = −13.17, df = 6, p < 0.001, indicating a statistically significant difference between pre- and post-instruction scores. Cohen’s d was 4.977, indicating a very large effect size in the group studied. These findings suggest that unplugged CT instruction may improve mathematics learning outcomes in plane geometry in micro-sized classrooms. However, the findings should be interpreted cautiously because the sample was small and the study had no control group.