Social networking sites (SNSs) are social platforms that facilitate communication. For adolescents, peers play a crucial role in constructing the self online through displays of group norms on SNSs. The current study investigated the role of online social identity (OSI) in the relationship between adolescent exposure to alcohol-related content posted by peers on SNSs and alcohol use. In a sample (N = 929) of Australian adolescents (Age M = 17.25, SD = 0.31) higher levels of exposure to alcohol-related content on SNSs was associated with higher levels of alcohol use. Importantly, the association was stronger when the participants reported higher OSI particularly when also reporting low or moderate amount of time spent on SNS. The findings can be explained by social identity literature that demonstrates individuals align their behaviors with other members of their social group to demonstrate, enact, and maintain social identity. The results of this study reflect the importance of considering the construction of the “self” through online and offline constructs.