Objective We aimed to assess the associations between problematic smartphone use, unhealthy behaviors, and mental health status among adolescents. Methods The data for this cross-sectional study were obtained from the 13th Korea Youth Risk Behavior Survey (2017). From among the 62,276 adolescents who participated in this nationwide web-based survey, 54,603 (87.7%) who used smartphones were selected as the study population. Unhealthy behaviors included smoking, drinking, physical inactivity, skipping breakfast, and fast food consumption. For the examination of mental health status, we considered sleep disturbances, stress, depressive symptoms, suicidal ideation, and suicide attempts. The associations between problematic smartphone use, unhealthy behaviors, and mental health were analyzed using multiple logistic regression analyses. Results More than four hours of use on weekdays (31.3%) and weekends (61.3%) and the use of smartphones for communication (46.9%) were associated with unhealthy behaviors and mental health problems. Furthermore, adolescents who experienced conflicts with family (22.3%) and friends (25.8%) and academic problems (3.4%) were likely to have health problems. Conclusion The results suggest that problematic smartphone use may disrupt adolescents’ health. Therefore, there is a need for prevention programs for problematic smartphone use at home and at school to improve adolescents’ health.