<![CDATA[This research was motivated by the low conceptual understanding of fourth-grade elementary students on flat-shape geometry, where 62.8% of students (42 out of 67) scored below the minimum proficiency standard, and instruction remained dominated by lecture methods without interactive digital media. The study aimed to develop and test the effectiveness of web-based interactive learning media to enhance students' conceptual understanding. The research employed a Research and Development (R&D) approach with a mixed-method design, integrating the ADDIE model (5 phases) and Borg & Gall (10 stages) into four main phases: preliminary study, planning, development, and implementation. Product validation was conducted by three experts (content, media, and instructional design), while effectiveness testing employed a pretest-posttest control group design involving 60 fourth-grade students (30 experimental, 30 control). Expert validation results achieved an average of 89.8% (highly valid), with content expert 87%, media expert 86.4%, and instructional design expert 96%. Small group trials (N=15) obtained 90.45% feasibility, and operational field trials (N=30) obtained 91.53% (highly feasible). Effectiveness results showed the experimental group achieved an average N-Gain of 83.44% (high/effective category), while the control group achieved only 53.08% (medium/moderate category). The Mann-Whitney U test on N-Gain scores yielded Z = -4.887, p < 0.001, indicating a significant difference between experimental and control groups. In conclusion, the web-based interactive learning media proved valid, feasible, and effective in improving fourth-grade students' conceptual understanding of flat shapes. The implications encourage teachers and schools to adopt interactive digital media as an innovative alternative in mathematics instruction aligned with the Merdeka Curriculum.]]>
