This study investigated the protective and risk factors of smartphone addiction at the individual and group levels using multi-level modelling, with a particular focus on extracurricular activities and academic helplessness. Data from the Korea Children and Youth Panel Survey (KCYPS) were used in the analyses. These data included 2,590 middle-school students (1,405 males and 1,185 females, 8th-grade students) nested in 17 cities. The findings revealed that individual-level variables, including lack of control belief and lack of active performance, were significant predictors of smartphone addiction. Furthermore, extracurricular activities contributed to decreased levels of smartphone addiction at both group and regional levels. The effects of individual-level variables on smartphone addiction differed depending on the rational-level variables. Based on these findings, educational implications and suggestions for future studies are discussed.