This study was conducted to investigate the moderated role of basic psychological needs by family cohesion in the relationship between shyness and mobile phone dependency among girl adolescents. It is a descriptive correlational research study. The study population included all girl adolescents aged 13 to 18, who were students in Tehran in 1401. 283 individuals were selected as a sample using the convenience sampling method and responded to the Revised Cheek and Buss Shyness Scale (Cheek, 1983), Mobile Phone Addiction Index (Leung, 2008), Basic Psychological Needs Scale (Deci & Ryan, 2000), and Family Organized Cohesiveness Scale (Fisher et al., 1992). Pearson correlation and multiple regression were used to analyze the data, and the Hayes macro-Process (model 4 & 59) in SPSS24 was installed. The results showed that basic psychological needs mediate the relationship between shyness and mobile phone dependency. The relation of shyness with mobile phone dependency and basic psychological needs, as well as the relationship between basic psychological needs and mobile phone dependence, is moderated by family cohesion. Based on this, by increasing the satisfaction of basic psychological needs in adolescents and increasing family cohesion, their shyness, and as a result, their dependence on mobile phones will decrease. These findings are useful for understanding the complex mechanism of shyness effects on mobile phone dependency and have practical implications for preventing and reducing adolescents' dependence on mobile phones. Keywords Shyness, Mobile Phone Dependency, Basic Psychological Needs, Family Cohesion, Girl Adolescents. Introduction Considering the expansion of mobile phone use among Iranian adolescents and the risk of dependency, it is necessary to understand the underlying mechanisms of mobile phone dependency to make decisions for preventive interventions. Personality characteristics, especially high levels of shyness, make adolescents vulnerable to mobile phone dependency (Han et al., 2017). Shy adolescents experience communication problems, social anxiety, loneliness, and depression (Liu et al., 2019). To avoid these negative emotions, they turn to excessive use of mobile phones, which leads to dependency (Enez et al., 2016). Although existing studies have shown the effect of shyness on mobile phone dependency, the mechanism of this relationship has been less investigated from the perspective of the interaction between individual and environmental factors. One of the individual factors related to mobile phone dependency is basic psychological needs. If basic psychological needs are not met, pathological compensation may occur. Shy people's withdrawn personality facilitates unsatisfied interpersonal relationships and unmet basic psychological needs in real situations. Anonymity and lack of face-to-face contact in the mobile network space provide a relatively safe and relaxing environment for shy people so that their unmet needs can be pathologically compensated by using mobile phones. Improper mobile phone use may lead to mobile phone dependency. This mediating process, in which shyness affects mobile phone dependency through basic psychological needs, may be influenced by the family as the primary direct microsystem. Studies have shown that high family cohesion is a protective factor against Internet addiction (Wang et al., 2018). Shy people may become dependent on mobile phones to escape from social pressures in reality and eliminate negative emotions. However, if family members have intimate relationships, these relationships as a source of social support can address some basic psychological needs of shy people and reduce the incidence of mobile phone dependency (Sela et al., 2019). At present, few studies have been conducted on the relationship between shyness and mobile phone dependency among adolescents. Adolescence is a period in which the body and mind undergo drastic changes and the level of shyness increases. Also, although the effect of shyness on mobile phone dependency has been examined only in some foreign studies, the mechanism of this relationship is not clear, and few studies have examined this relationship from the perspective of the interaction between the person and the environment (Hong et al., 2019; Sela et al., 2019). Based on this, considering the expansion of mobile phone use, it is necessary to identify the impact that personal and individual characteristics and environmental factors have on mobile phone dependency in this period. Therefore, this research aims to investigate the mediating role of basic psychological needs in the relationship between shyness and mobile phone dependency in adolescent girls and to study the moderating role of family cohesion in the mediation process. Methodology The method of this research is descriptive and correlational. The statistical population included all adolescent girls aged 13 to 18 who were students in Tehran in 1401. A sample of 283 was selected using a convenience sampling method and responded to the Revised Cheek and Buss Shyness Scale (Cheek, 1983), Mobile Phone Addiction Index (Leung, 2008), Basic Psychological Needs Scale (Deci & Ryan, 2000) & Family Organized Cohesiveness Scale (Fisher et al., 1992). Descriptive Pearson’s correlation and multiple regression were used to analyze the data using the Hayes macro-process (models 4 & 59) in SPSS24. The moderated mediation model was also tested with 5000 bootstrap samples and a 95% confidence interval. Findings Investigating mediation paths using the Hayes regression model 4 showed that shyness has a positive and significant relationship with mobile phone dependency and a negative and significant relationship with basic psychological needs. Also, the relationship between basic psychological needs and mobile phone dependency was found to be negative and significant. When all three variables were entered into the regression equation, shyness positively and significantly predicted mobile phone dependency. Then, the indirect path of the relationship between shyness and mobile phone dependency was significant through the mediation of basic psychological needs. The bootstrap test results indicated the indirect effect of basic psychological needs with a 95% confidence interval, which means the mediation total effect was estimated as 57.90%. Examining the moderating paths using the 59 Hayes regression model showed that the interaction of shyness with family cohesion significantly predicted basic psychological needs and mobile phone dependency. Also, the interaction of basic psychological needs with family cohesion significantly predicted mobile phone dependency. Result The findings showed that shyness is directly and indirectly related to mobile phone dependency through basic psychological needs, and the mediation of basic psychological needs is moderated by family cohesion. In other words, the relationship of shyness with mobile phone dependency and basic psychological needs and the relationship between basic psychological needs and mobile phone dependency were all moderated by family cohesion. The findings contribute to a deeper understanding of the relationship between shyness and mobile phone dependency. As a practical suggestion, attention should be paid to the effect of shyness on mobile phone dependency, particularly for people with high shyness, to strengthen intervention and support and reduce the likelihood of mobile phone dependency through effective control of the level of shyness. Finally, understanding and satisfying the basic psychological needs of shy people through social skills training and self-disclosure tasks to improve social skills is important to reduce mobile phone dependency, and it is necessary to understand the importance of family cohesion in supporting individuals.