Bangun Ruang AR 3D is an Augmented Reality technology-based application designed to help elementary school students understand spatial geometry material visually and interactively. This application is built using Unity 3D with Vuforia SDK integration, and displays five main 3D spatial geometry objects, namely cubes, blocks, spheres, cylinders, and cones. Users simply point the camera at the marker image to display the spatial geometry object in three dimensions, which can be rotated and observed from various angles. Each object is equipped with a formula button to display surface area and volume information directly. In addition, this application has a Mini Quiz feature that allows students to test their understanding through multiple-choice questions arranged according to the elementary school curriculum. With a simple and child-friendly interface design, this application is expected to increase students' interest in learning and make it easier for teachers to deliver material. Application development follows the Multimedia Development Life Cycle (MDLC) method to ensure the quality and functionality of each feature. Users can start by opening the main page of the application which displays several menu buttons, namely Play AR, Mini Quiz, Information, and Exit. To learn spatial geometry, users select the Play AR button, then point the device camera at the printed spatial geometry marker. A 3D object will appear on the screen, and users can press the “Formula” button to see the area and volume calculations. To test their understanding, users can select the Mini Quiz menu which contains interactive questions with automatic scoring. The Information menu will display an explanation of the application and the developer. Once finished, users can press the Exit button to close the application.