Background: Mental health is considered as an integral and essential component of overall health. Its determinants and related factors are one of the most important research priorities, especially in adolescents and young people. Using a qualitative approach, the present study aimed to identify factors affecting the mental health of youth in Iran. Methods: In 2017, following content analysis principles, and using semi-structured in-depth interviews, we conducted a qualitative study exploring the opinions of young people about mental health. A targeted sampling method was used, and participants were young volunteers aged 18 to 30 who were selected from Tehran province, Iran. Inclusion criteria for participants was willingness to participate in the study, and ability to express their experiences. Data collection was done with individual in-depth interviews. According to the explanatory model, the interviews were directed toward the concept of mental health and path of causality and auxiliary behaviors. Results: 21 young adults participated, who met the study inclusion criteria, of whom 12 participants were male. Their mean age was 24.4 ± 0.41 years and their education varied from primary school to Master’s degree. Mental health was considered as mental well-being and a sense of satisfaction and efficacy, not only the presence of a disease or mental disorder. Based on the opinions of the interviewees, three factors of personal characteristics, family and society are involved in mental health. Individual factors were associated with behavioral and physical problems. One of the most important issues was revealed as tensions in societal and family conflicts. Economic problems and unemployment of young people were also extracted from the social factor. Conclusion: In Iran, social factors such as jobs for the unemployed and job security are considered as important determinants in the mental health of young people.