The purpose of the study was to investigate the effect of inquiry-based instruction enriched with origami activities on 7th grade students' achievement in reflection symmetry and self-efficacy in geometry. Two classes, instructed by the first author of the paper, were randomly assigned as experimental and control groups. In order to gather data, participants were administered Reflection Symmetry Achievement Test, and Geometry Self-Efficacy Scale as pre-test and post-test. The Analysis of Covariance was performed in order to answer the research questions. Moreover, five participants were interviewed to examine self-efficacy sources which are determinant of the change in self-efficacy levels. Findings revealed that the inquiry-based instruction enriched with origami activities had a significantly positive effect on students' achievement in reflection symmetry and self-efficacy in geometry. Interviews showed that all four sources were influenced by the intervention.