This study investigated the causal relationships between quality of social relationships and smartphone addiction in high school students. The research used exploratory sequential mixed design and was carried out in twostages: qualitative and quantitative. In the qualitative stage, a focus group interview was conducted to examinestudents’ views on the relationship between quality of social relationships and smartphone addiction. In thequantitative stage, a survey was conducted, using questions developed from findings obtained in the qualitativestage and various measurement tools. Participants included 11th and 12th graders. Eight participants recruitedbased on smartphone use took part in the qualitative stage, while 620 students were recruited on a voluntary-basis for the quantitative stage. An interview form developed by the authors was used in the qualitativestage, while the Smartphone Addiction Scale-Short Version, UCLA Loneliness Scale-Short Form, Perceptionsof Parents Scale, and Social Self-Efficacy Scale were used in the quantitative stage. Content analysis was carried out on qualitative data, and multiple linear regression was conducted on quantitative data. The findingsrevealed that smartphone addiction was positively associated with loneliness, multipurpose smartphone use,social media, and having nothing to do, and negatively associated with perception of one’s mother, perceptionof one’s father, and spending quality time with friends. Social media had the highest contribution to the variance in smartphone addiction.