The purpose of this study by Ju Yeon Kwak, Jae Yop Kim, and Yoe Won Yoon was to investigate the importance of the relationships with parents, peers, and teachers as a cause of adolescents’ smartphone addiction, and to examine the effect of parental neglect on smartphone addiction and the mediating effect of relational maladjustment in school, especially focusing on the relational maladjustment with peers and teachers. A total of 1170 middle-school students who reported using smartphone took part in this study. Parental neglect was significantly associated with adolescents’ smartphone addiction. Furthermore, in the relationship between parental neglect and smartphone addiction, parental neglect was not significantly associated with the relational maladjustment with peers, whereas the relational maladjustment with peers negatively influenced smartphone addiction. On the other hand, the relational maladjustment with teachers had a partial mediation effect between parental neglect and smartphone addiction. Based on the results of this study, some implications are suggested that include the need for (1) a customized program for adolescents who use smartphones addictively, (2) a family therapy program to strengthen family function, (3) an integrated case-management system to prevent the reoccurrence of parental neglect, (4) a program to improve relationships with teachers, and (5) expanding the leisure activity infrastructure to improve relationships with friends off-line.