Using data from the seventh wave of the World Values Survey, this study examines how social media use shapes the negotiation of transnational identity in South Korea and Taiwan—two countries characterized by distinct forms of nationalism—and whether social capital moderates this relationship. A two-dimensional model encompassing local–global attachment and protectionism–openness was employed to identify latent classes of transnational identity. The findings reveal that nationalism significantly influences identity negotiation in both countries, with Taiwanese citizens exhibiting stronger global ties compared to Koreans. In addition, social media use reinforces local and protectionist identities in these countries, limiting transnational solidarity and amplifying exclusivity through algorithmically mediated digital networks. Notably, social capital moderates this dynamic by fostering global openness, particularly when trust in out-groups is high. This study highlights the intricate interplay between social media use, social capital, and transnational identity negotiation, contributing to a nuanced understanding of the cultural response to globalization in East Asia.