The purpose of this study was to investigate the effectiveness of the modified version of the smartphone addiction prevention program developed by the National Information Society Agency. Upper elementary school students were recruited from community youth centers located in Chungju. A total of 86 students agreed to participate in the study; 42 students were given the 8-session smartphone addiction prevention program (experimental group) and 44 students were placed on the wait-list (control group). The prevention program addressed cognitions and emotions precipitating smartphone addiction and included a plan for a wise smartphone use and a plan for activities and behaviors as alternatives to smartphone use. The average daily use time for smartphone and levels of smartphone addiction, anxiety, depression, impulsivity, and aggression were measured at pre- and post-study points both in the experimental and control groups. Two-way ANOVAs with group (experiment and control groups) and measurement point (pre & post) as independent variables revealed significant interaction effects for all six outcomes. Further analyses testing pre-post score differences in each group showed a significant pre-post decrease in all six outcomes of the experimental group, but not in the wait-list control group. These results demonstrate the effectiveness of the smartphone addiction prevention program not only in decreasing upper elementary school students’ overuse of and addiction to the smartphone but also in improving their mental health in areas of anxiety, depression, impulsivity, and aggression.