Social media is used for political influence, but do digital advertisements have the power to shape how users interpret the sociopolitical messages that are shared through social media? In 2 experiments (ns = 86 and 225), we tested whether digital advertisements have the capacity to act as identity signals, affecting the degree to which the source of a sociopolitical message is perceived as an outgroup—rather than ingroup—member. We also examined whether these perceptions predict one form of sociopolitical behavior: solidarity-based collective action. Participants viewed an online blog that asked them to take collective action to support an outgroup. Simultaneously, ostensibly incidental banner advertisements were presented alongside the blog. In Study 1, these adverts related to either the outgroup, superordinate category, or neither category: Compared with control, digital advertisements relating to the outgroup led to a greater likelihood that the message source would be categorized as an outgroup member. In Study 2, the adverts related to either the outgroup, ingroup, or neither: Adverts relating to the ingroup led to reduced likelihood that the source would be categorized as an outgroup member, relative to adverts relating to the outgroup. In Study 1, social categorization of the message source, in turn, predicted solidarity-based collective action, moderated by social identification with the outgroup; however, there was no such association in Study 2. Our findings contribute to debates about the impact of social media on democracy, and the importance of transparency and accountability in how social media environments are curated.