Geometry students have few opportunities to apply geometric concepts and solve geometry problems in authentic contexts. We developed a Ubiquitous Geometry (UG) system to teach geometry to elementary students by using it to measure common objects in authentic contexts. The instant study investigates the effects of UG on student geometry estimation ability, geometry learning achievement, spatial ability, and learning geometric concepts and explores the correlations. Participants were 82 fifth grade students divided into the experiment group (EG), the ruler measurement group (RG), and the control group (CG). The results revealed that the EG students performed significantly better than the others in geometry estimation ability, geometry learning achievement, spatial ability, and understanding geometric concepts. We found that geometry estimation ability is significantly related to spatial ability and geometry learning achievement, which means fostering estimation ability through measuring authentic objects could help students to learn geometry. Finally, students perceived that UG engaged them more in geometry and improved their interest in learning because the activity designs related more to their daily life, thereby enhancing their sense of confidence and satisfaction. Conclusions include that UG effectively enhances student geometry estimation ability, geometry learning achievement, spatial ability, and understanding of geometric concepts.