The purpose of this study was to examine the effects of using GeoGebra software in a computer-supported collaborative learning (CSCL) environment on seventh grade students' geometry achievement, retention of learning, and attitudes toward geometry. The study was designed using a quasi-experimental research method with pretest, post-test and delayed post-test. This study was carried out with 62 seventh grade students in a city in western Turkey. CSCL activities using GeoGebra software were implemented in the experimental group, while instruction in the control group continued with textbook-based direct instruction. The Geometry Achievement Test (GAT) and Geometry Attitude Scale (GAS) were applied to groups as pretest and post-test. A retention test was applied to both groups eight weeks after the post-test. Data were analyzed through SPSS 17.0 statistical software by using a t-test and ANCOVA test. It was indicated in this study that CSCL using GeoGebra software significantly increased seventh grade students' geometry achievement and retention of learning in comparison to textbook-based direct instruction. It was also determined that the CSCL environment with GeoGebra software significantly increased students' attitudes toward geometry.