<![CDATA[Purpose: This study addresses Grade XII students' visualization difficulties in 3D spatial geometry and mitigates traditional, mechanistic teaching practices by developing a Realistic Mathematics Education (RME) module integrated with dynamic GeoGebra software. Design/methodology/approach: Utilizing Plomp’s educational design research framework, a Hypothetical Learning Trajectory (HLT) was implemented across six instructional sessions. Empirical data were gathered from 34 Grade XII students and two mathematics teachers using expert validation rubrics, 5-point Likert practicality questionnaires, qualitative field notes, and spatial reasoning pre/post-tests. Findings: Expert validation confirmed high content validity. The module achieved exceptional practicality scores of 91.5% from teachers and 87.4% from students. Inferential testing demonstrated a statistically significant improvement in spatial problem-solving performance ($t(33) = 14.82, p < .001$), accompanied by heightened peer collaboration and dynamic 3D model manipulation. Practical implications: Provides educators with an actionable, scaffolded framework to transition from teacher-centered lecturing toward guided reinvention, simplifying complex 3D perpendicularity concepts without relying on static chalkboards. Originality/value: Offers a novel synthesis of RME principles and dynamic 3D visual technology specifically tailored for secondary spatial geometry, resolving persistent pedagogical constraints in mathematics classrooms. Purpose: Penelitian ini mengurasi kendala visualisasi geometri ruang 3D pada siswa Kelas XII dan mengatasi metode pengajaran mekanistik guru melalui pengembangan modul Realistic Mathematics Education (RME) terintegrasi perangkat lunak dinamis GeoGebra. Design/methodology/approach: Menggunakan kerangka educational design research Plomp, Hypothetical Learning Trajectory (HLT) diimplementasikan selama enam sesi pembelajaran. Data empiris dikumpulkan dari 34 siswa Kelas XII dan dua guru matematika menggunakan rubrik validasi ahli, angket kepraktisan skala Likert 5 poin, catatan lapangan kualitatif, serta pre/post-test penalaran spasial. Findings: Validasi ahli mengonfirmasi validitas isi yang tinggi. Modul mencapai tingkat kepraktisan luar biasa sebesar 91,5% dari guru dan 87,4% dari siswa. Uji inferensial menunjukkan peningkatan signifikan pada kemampuan pemecahan masalah spasial ($t(33) = 14,82; p < 0,001$) yang diiringi penguatan kolaborasi antar-siswa dan manipulasi model 3D dinamis. Practical implications: Menyediakan panduan ber-scaffolding bagi pendidik untuk bertransisi dari instruksi berpusat pada guru menuju penemuan terbimbing (guided reinvention), mempermudah konseptualisasi ketegaklurusan 3D tanpa bergantung pada papan tulis statis. Originality/value: Memberikan kontribusi berupa sintesis baru antara prinsip RME dan teknologi visualisasi 3D dinamis yang dirancang khusus untuk geometri ruang SMA guna menjawab hambatan pedagogis dalam pembelajaran matematika.]]>
