This research study examined the effect of origami-based geometry instruction on spatial visualization, geometry achievement, and geometric reasoning of tenth-grade students in Turkey. The sample ("n" = 184) was chosen from a tenth-grade population of a public high school in Turkey. It was a quasi-experimental pretest/posttest design. A control group (94 students) received regular instruction during a geometry unit in a tenth-grade classroom whereas an experimental group (90 students) received origami-based instruction for 4 weeks. The Spatial Visualization Test (SVT) was used to measure students' spatial visualization ability in this study. The SVT consists of the Card Rotation Test, Cube Comparison Test, and the Paper Folding Test which were originally developed by Ekstrom, French, Harman & Derman (1976) and translated into Turkish by Delialioglu (1996). Besides, Geometry Achievement Test and Geometric Reasoning Test were developed by the researcher to measure geometry achievement level and geometric reasoning level of participants in key aspects of triangles. All tests had versions of pretest and posttest, and these tests were administered to both groups. A repeated-measures Analysis of Variance was used on each test scores to analyze data. The results indicated that origami-based instruction had significant effect on all dependent variables (spatial visualization, geometry achievement, and geometric reasoning). This suggested that origami might be integrated into high school geometry lessons to make geometry learning more effective.