The world’s largest transnational fandom community of the BTS (BangTan Sonyeondan) ARMY (Adorable Representative M.C. for Youth) is structured to achieve common goals and exert influence. To identify a mechanism that explains the organizational structure of transnational virtual communities, we empirically investigate how the global fandoms carry out autonomous activities and adapt to the environment without coercive governance despite their massive size and complexity. Using the techniques of social network analysis and viable system model, we found some structural differences between the online communities with a viable and non-viable structure. First, the BTS ARMY network is sustained by a systematic structure of key players who voluntarily play distinct roles. In contrast, the Arianator network is relatively unorganized and dispersed. Moreover, multi-fandom accounts can act as potential user acquisition roles. Finally, two different diffusion types, text-based information and image-based media, are carried by distinct roles of subsystems. As one of the first studies to empirically examine the viability of online transnational fandom from an organizational structure perspective, this paper provides practical implications for fandoms from three perspectives; (1) resource mobilization and sustainability, (2) influence of fandom dynamics, and (3) economic impact. The results suggest that fandoms taking strategic actions upon the viable structure on digital platforms could represent a new cultural hegemony, promoting global unity and pop cosmopolitanism.