Fear of missing out (FoMO) refers to the desire to stay continually connected with what others are doing because they are afraid of missing the flow or not being included. Although FoMO has been recently proposed as a widespread phenomenon within social media, there has been few empirical research by which mechanism FoMO leads to social networking service (SNS) addiction. Therefore, this study examined whether FoMO mediated the relationship between adolescents’ attachment and SNS addiction proneness. A total of 517 middle and high school students participated in the study and completed self-report questionnaires. Results showed that FoMO partially mediated the relationship between adolescents’ parental attachment and SNS addiction proneness, whereas the relationship between adolescents’ peer attachment and SNS addiction proneness was fully mediated by FoMO. These results suggest that unstable attachment might contribute to SNS addiction through FoMO, especially in case of peer attachment. Lastly, we discussed theoretical and practical implications of this study and suggestions for future research.