Considering the destructive and growing effects of the Internet addiction in today's world, this study has been undertaken to investigate the relationship of metacognitive beliefs and emotional security with Internet addiction in adolescent girls. This research had a correlational-descriptive design. The research population of the study was comprised of senior high school female students in Tabriz in 2016-17 school year from among whom 252 individuals were selected based on multi-stage cluster sampling method. The research instruments included Meta-Cognitions Questionnaire - Adolescent Version (MCQ-A), Belonging and Emotional Security Tool (BEST), and Young’s Addiction to Internet Test (IAT). Pearson correlation coefficients showed that there was a significant, positive relationship between the metacognition total score and its two subscales- thoughts concerning uncontrollability and danger- and cognitive confidence with Internet addiction (p<0.01), whereas there was a significant, negative relationship between emotional security total score and its two subscales - safety and belonging (p<0.01). The results of regression analysis revealed that thoughts concerning uncontrollability and danger could predict and explain variance of the Internet addiction significantly and positively, while safety could predict and explain it significantly and negatively. Based on the results of this study, it can be concluded that in order to reduce the tendency of adolescents to use the Internet, it is possible to change dysfunctional metacognitive beliefs and teach the main functions of the family and its effects on teenagers' emotional safety.