Literature highlighted that problematic social network site use might have relevant detrimental consequences on users’ well-being, particularly in adolescence. However, it is still not clear which variables could be considered protective or risk factors for such problematic use. The present study aimed to fill this gap, examining the relationship between reflective functioning and adolescents’ identity development, while taking problematic SNS use into account. A sample of 354 students (age range = 13–19; Mage = 16.18, SD = 1.58; 19.9% males) from two Italians high schools participated in the study. Results from structural equation modeling showed that adolescents with high levels of reflective functioning reported lower levels of problematic SNS use. Furthermore, problematic SNS use mediated the relationship between reflective functioning and identity development, by disfavouring identity in-depth exploration and favouring reconsideration of commitment. We discussed clinical and research implications.