Spatial ability is a critical foundation for geometry learning; however, teaching solid geometry in elementary schools still largely relies on static two-dimensional representations, making it difficult for students to mentally visualize three-dimensional objects. This study aims to develop an innovative mathematics learning medium that integrates gamification and Augmented Reality (AR) in solid geometry content to improve elementary school students' spatial ability and to examine its validity, practicality, and effectiveness. The study used a Research and Development (R&D) approach based on the ADDIE model (Analysis, Design, Development, Implementation, Evaluation). A medium named "RuangKu AR" was developed by integrating AR-based 3D visualization of solid shapes with gamification elements (points, levels, badges, and leaderboards) designed on the basis of Self-Determination Theory to encourage active student engagement. Development results show that content experts, media experts, and language experts rated the medium as highly valid (average 92%), and teachers (91%) and students (89%) rated it as highly practical. The effectiveness test, conducted as a field trial using a one-group pretest–posttest design involving 30 elementary school students, showed that the medium effectively improved students' spatial ability (mean score increased from 58.4 to 84.7; N-Gain of 0.63, in the medium category). This study contributes an alternative mathematics learning medium that is contextual, interactive, and relevant to twenty-first-century learning needs, while also offering a solution to the limited availability of representative physical teaching aids in elementary schools.