This study developed and evaluated a GeoGebra-assisted instructional tool in teaching key concepts of circles to Grade 10 students. The development of the instructional tool followed the ADDIE model and was assessed by evaluators based on its instructional, content, and technical quality. The evaluations indicated that the tool is of excellent quality. A quasi-experimental approach was then employed to measure the tool’s effectiveness in improving the performance of students. The study involved two heterogeneous Grade 10 sections from Koronadal National Comprehensive High School (KNCHS). Effectiveness was determined by comparing the pretest and posttest scores of control and experimental groups using an Independent t-Test. The results revealed that the GeoGebra-assisted instructional tool significantly improved students' understanding of key concepts of circle. Based on these findings, it is recommended that other mathematics teachers adopt this tool to enhance student learning. Additionally, further exploration of GeoGebra's features is encouraged.